HTML5

This specification defines the 5th major revision of the core language of the World Wide Web: the Hypertext Markup Language (HTML). In this version, new features are introduced to help Web application authors, new elements are introduced based on research into prevailing authoring practices, and special attention has been given to defining clear conformance criteria for user agents in an effort to improve interoperability.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the most recently formally published revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Implementors should be aware that this specification is not stable. Implementors who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation stage should join the aforementioned mailing lists and take part in the discussions.

The publication of this document by the W3C as a W3C Working Draft does not imply that all of the participants in the W3C HTML working group endorse the contents of the specification. Indeed, for any section of the specification, one can usually find many members of the working group or of the W3C as a whole who object strongly to the current text, the existence of the section at all, or the idea that the working group should even spend time discussing the concept of that section.

The W3C HTML Working Group is the W3C working group responsible for this specification's progress along the W3C Recommendation track. This specification is the 24 November 2009 Editor's Draft.

This specification is also being produced by the WHATWG. The two specifications are identical from the table of contents onwards.

This specification is intended to replace (be a new version of) what was previously the HTML4, XHTML1, and DOM2 HTML specifications.

1 Introduction

1.1 Background

The World Wide Web's markup language has always been HTML. HTML was primarily designed as a language for semantically describing scientific documents, although its general design and adaptations over the years have enabled it to be used to describe a number of other types of documents.

The main area that has not been adequately addressed by HTML is a vague subject referred to as Web Applications. This specification attempts to rectify this, while at the same time updating the HTML specifications to address issues raised in the past few years.

1.2 Audience

This specification is intended for authors of documents and scripts that use the features defined in this specification, implementors of tools that operate on pages that use the features defined in this specification, and individuals wishing to establish the correctness of documents or implementations with respect to the requirements of this specification.

This document is probably not suited to readers who do not already have at least a passing familiarity with Web technologies, as in places it sacrifices clarity for precision, and brevity for completeness. More approachable tutorials and authoring guides can provide a gentler introduction to the topic.

In particular, familiarity with the basics of DOM Core and DOM Events is necessary for a complete understanding of some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML, Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not essential.

1.3 Scope

This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the Web ranging from static documents to dynamic applications.

The scope of this specification does not include providing mechanisms for media-specific customization of presentation (although default rendering rules for Web browsers are included at the end of this specification, and several mechanisms for hooking into CSS are provided as part of the language).

The scope of this specification is not to describe an entire operating system. In particular, hardware configuration software, image manipulation tools, and applications that users would be expected to use with high-end workstations on a daily basis are out of scope. In terms of applications, this specification is targeted specifically at applications that would be expected to be used by users on an occasional basis, or regularly but from disparate locations, with low CPU requirements. For instance online purchasing systems, searching systems, games (especially multiplayer online games), public telephone books or address books, communications software (e-mail clients, instant messaging clients, discussion software), document editing software, etc.

1.4 History

For its first five years (1990-1995), HTML went through a number of revisions and experienced a number of extensions, primarily hosted first at CERN, and then at the IETF.

With the creation of the W3C, HTML's development changed venue again. A first abortive attempt at extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as HTML 3.2, which was completed in 1997. HTML4 followed, reaching completion in 1998.

At this time, the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C also worked on a new language that was not compatible with the earlier HTML and XHTML languages, calling it XHTML2.

Around the time that HTML's evolution was stopped in 1998, parts of the API for HTML developed by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then petered out, with some DOM Level 3 specifications published in 2004 but the working group being closed before all the Level 3 drafts were completed.

In 2003, the publication of XForms, a technology which was positioned as the next generation of Web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements for it. This interest was borne from the realization that XML's deployment as a Web technology was limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for existing deployed technologies (like HTML).

A proof of concept to show that it was possible to extend HTML4's forms to provide many of the features that XForms 1.0 introduced, without requiring browsers to implement rendering engines that were incompatible with existing HTML Web pages, was the first result of this renewed interest. At this early stage, while the draft was already publicly available, and input was already being solicited from all sources, the specification was only under Opera Software's copyright.

The idea that HTML's evolution should be reopened was tested at a W3C workshop in 2004, where some of the principles that underlie the HTML5 work (described below), as well as the aforementioned early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the Web's evolution; the W3C staff and membership voted to continue developing XML-based replacements instead.

Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specification.

The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.

The latter requirement in particular required that the scope of the HTML5 specification include what had previously been specified in three separate documents: HTML4, XHTML1, and DOM2 HTML. It also meant including significantly more detail than had previously been considered the norm.

In 2006, the W3C indicated an interest to participate in the development of HTML5 after all, and in 2007 formed a working group chartered to work with the WHATWG on the development of the HTML5 specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.

A separate document has been published by the W3C HTML working group to document the differences between this specification and the language described in the HTML4 specification. [HTMLDIFF]

1.5 Design notes

It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.

HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.

Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the Web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.

Despite all this, efforts have been made to adhere to certain design goals. These are described in the next few subsections.

1.5.1 Serializability of script execution

To avoid exposing Web authors to the complexities of multithreading, the HTML and DOM APIs are designed such that no script can ever detect the simultaneous execution of other scripts. Even with workers, the intent is that the behavior of implementations can be thought of as completely serializing the execution of all scripts in all browsing contexts.

The navigator.yieldForStorageUpdates() method, in this model, is equivalent to allowing other scripts to run while the calling script is blocked.

1.5.2 Compliance with other specifications

This specification interacts with and relies on a wide variety of other specifications. In certain circumstances, unfortunately, the desire to be compatible with legacy content has led to this specification violating the requirements of these other specifications. Whenever this has occurred, the transgressions have each been noted as a "willful violation".

1.6 HTML vs XHTML

This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.

The in-memory representation is known as "DOM HTML", or "the DOM" for short. This specification defines version 5 of DOM HTML, known as "DOM5 HTML".

There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.

The first such concrete syntax is the HTML syntax. This is the format suggested for most authors. It is compatible with most legacy Web browsers. If a document is transmitted with the MIME type text/html, then it will be processed as an HTML document by Web browsers. This specification defines version 5 of the HTML syntax, known as "HTML5".

The second concrete syntax is the XHTML syntax, which is an application of XML. When a document is transmitted with an XML MIME type, such as application/xhtml+xml, then it is treated as an XML document by Web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML from being rendered fully, whereas they would be ignored in the HTML syntax. This specification defines version 5 of the XHTML syntax, known as "XHTML5".

The DOM, the HTML syntax, and XML cannot all represent the same content. For example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM and in XML. Similarly, documents that use the noscript feature can be represented using the HTML syntax, but cannot be represented with the DOM or in XML. Comments that contain the string "-->" can be represented in the DOM but not in the HTML syntax or in XML.

1.7 Structure of this specification

1.7.1 How to read this specification

This specification should be read like all other specifications. First, it should be read cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be read by picking random sections from the contents list and following all the cross-references.

1.7.2 Typographic conventions

The defining instance of a term is marked up like this. Uses of that term are marked up like this or like this.

The defining instance of an element, attribute, or API is marked up like this. References to that element, attribute, or API are marked up like this.

1.8 A quick introduction to HTML

HTML documents consist of a tree of elements and text. Each element is denoted in the source by a start tag, such as "<body>", and an end tag, such as "</body>". (Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)

Tags have to be nested such that elements are all completely within each other, without overlapping:

This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.

Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink, formed using the a element and its href attribute:

Attributes are placed inside the start tag, and consist of a name and a value, separated by an "=" character. The attribute value can be left unquoted if it doesn't contain spaces or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes. The value, along with the "=" character, can be omitted altogether if the value is the empty string.

HTML user agents (e.g. Web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.

DOM trees contain several kinds of nodes, in particular a DOCTYPE node, elements, text nodes, and comment nodes.

The root element of this tree is the html element, which is the element always found at the root of HTML documents. It contains two elements, head and body, as well as a text node between them.

There are many more text nodes in the DOM tree than one would initially expect, because the source contains a number of spaces (represented here by "␣") and line breaks ("⏎") that all end up as text nodes in the DOM.

The head element contains a title element, which itself contains a text node with the text "Sample page". Similarly, the body element contains an h1 element, a p element, and a comment.

This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript) are small programs that can be embedded using the script element or using event handler content attributes. For example, here is a form with a script that sets the value of the form's output element to say "Hello World":

Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link (e.g. the a element in the tree above) can have its "href" attribute changed in several ways:

Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations (especially interactive implementations like Web browsers), this specification is mostly phrased in terms of DOM trees, instead of the markup described above.

HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display. To influence exactly how such rendering takes place, authors can use a styling language such as CSS.

For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first.

1.9 Recommended reading

2 Common infrastructure

2.1 Terminology

This specification refers to both HTML and XML attributes and IDL attributes, often in the same context. When it is not clear which is being referred to, they are referred to as content attributes for HTML and XML attributes, and IDL attributes for those defined on IDL interfaces. Similarly, the term "properties" is used for both JavaScript object properties and CSS properties. When these are ambiguous they are qualified as object properties and CSS properties respectively.

Generally, when the specification states that a feature applies to the HTML syntax or the XHTML syntax, it also includes the other. When a feature specifically only applies to one of the two languages, it is called out by explicitly stating that it does not apply to the other format, as in "for HTML, ... (this does not apply to XHTML)".

This specification uses the term document to refer to any use of HTML, ranging from short static documents to long essays or reports with rich multimedia, as well as to fully-fledged interactive applications.

For simplicity, terms such as shown, displayed, and visible might sometimes be used when referring to the way a document is rendered to the user. These terms are not meant to imply a visual medium; they must be considered to apply to other media in equivalent ways.

When an algorithm B says to return to another algorithm A, it implies that A called B. Upon returning to A, the implementation must continue from where it left off in calling B.

2.1.1 Resources

The specification uses the term supported when referring to whether a user agent has an implementation capable of decoding the semantics of an external resource. A format or type is said to be supported if the implementation can process an external resource of that format or type without critical aspects of the resource being ignored. Whether a specific resource is supported can depend on what features of the resource's format are in use.

For example, a PNG image would be considered to be in a supported format if its pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image also contained animation data.

A MPEG4 video file would not be considered to be in a supported format if the compression format used was not supported, even if the implementation could determine the dimensions of the movie from the file's metadata.

The term MIME type is used to refer to what is sometimes called an Internet media type in protocol literature. The term media type in this specification is used to refer to the type of media intended for presentation, as used by the CSS specifications. [RFC2046] [MQ]

A string is a valid MIME type if it matches the media-type rule defined in section 3.7 "Media Types" of RFC 2616. [HTTP]

2.1.2 XML

To ease migration from HTML to XHTML, UAs conforming to this specification will place elements in HTML in the http://www.w3.org/1999/xhtml namespace, at least for the purposes of the DOM and CSS. The term "HTML elements", when used in this specification, refers to any element in that namespace, and thus refers to both HTML and XHTML elements.

Except where otherwise stated, all elements defined or mentioned in this specification are in the http://www.w3.org/1999/xhtml namespace, and all attributes defined or mentioned in this specification have no namespace.

Attribute names are said to be XML-compatible if they match the Name production defined in XML, they contain no U+003A COLON characters (:), and their first three characters are not an ASCII case-insensitive match for the string "xml". [XML]

The term XML MIME type is used to refer to the MIME types text/xml, application/xml, and any MIME type whose subtype ends with the four characters "+xml". [RFC3023]

2.1.3 DOM trees

The term root element, when not explicitly qualified as referring to the document's root element, means the furthest ancestor element node of whatever node is being discussed, or the node itself if it has no ancestors. When the node is a part of the document, then that is indeed the document's root element; however, if the node is not currently part of the document tree, the root element will be an orphaned node.

The Document of a Node (such as an element) is the Document that the Node's ownerDocument IDL attribute returns.

When an element's root element is the root element of a Document, it is said to be in a Document. An element is said to have been inserted into a document when its root element changes and is now the document's root element. Analogously, an element is said to have been removed from a document when its root element changes from being the document's root element to being another element.

If a Node is in a Document then that Document is always the Node's Document, and the Node's ownerDocument IDL attribute thus always returns that Document.

The term tree order means a pre-order, depth-first traversal of DOM nodes involved (through the parentNode/childNodes relationship).

When it is stated that some element or attribute is ignored, or treated as some other value, or handled as if it was something else, this refers only to the processing of the node after it is in the DOM. A user agent must not mutate the DOM in such situations.

The term text node refers to any Text node, including CDATASection nodes; specifically, any Node with node type TEXT_NODE (3) or CDATA_SECTION_NODE (4). [DOMCORE]

A content attribute is said to change value only if its new value is different than its previous value; setting an attribute to a value it already has does not change it.

2.1.4 Scripting

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".

An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.

If a DOM object is said to be live, then that means that any attributes returning that object must always return the same object (not a new object each time), and the attributes and methods on that object must operate on the actual underlying data, not a snapshot of the data.

The terms fire and dispatch are used interchangeably in the context of events, as in the DOM Events specifications. [DOMEVENTS]

2.1.5 Plugins

The term plugin is used to mean any content handler for Web content types that are either not supported by the user agent natively or that do not expose a DOM, which supports rendering the content as part of the user agent's interface.

One example of a plugin would be a PDF viewer that is instantiated in a browsing context when the user navigates to a PDF file. This would count as a plugin regardless of whether the party that implemented the PDF viewer component was the same as that which implemented the user agent itself. However, a PDF viewer application that launches separate from the user agent (as opposed to using the same interface) is not a plugin by this definition.

This specification does not define a mechanism for interacting with plugins, as it is expected to be user-agent- and platform-specific. Some UAs might opt to support a plugin mechanism such as the Netscape Plugin API; others might use remote content converters or have built-in support for certain types. [NPAPI]

Browsers should take extreme care when interacting with external content intended for plugins. When third-party software is run with the same privileges as the user agent itself, vulnerabilities in the third-party software become as dangerous as those in the user agent.

2.1.6 Character encodings

The preferred MIME name of a character encoding is the name or alias labeled as "preferred MIME name" in the IANA Character Sets registry, if there is one, or the encoding's name, if none of the aliases are so labeled. [IANACHARSET]

An ASCII-compatible character encoding is a single-byte or variable-length encoding in which the bytes 0x09, 0x0A, 0x0C, 0x0D, 0x20 - 0x22, 0x26, 0x27, 0x2C - 0x3F, 0x41 - 0x5A, and 0x61 - 0x7A, ignoring bytes that are the second and later bytes of multibyte sequences, all correspond to single-byte sequences that map to the same Unicode characters as those bytes in ANSI_X3.4-1968 (US-ASCII). [RFC1345]

This includes such encodings as Shift_JIS, HZ-GB-2312, and variants of ISO-2022, even though it is possible in these encodings for bytes like 0x70 to be part of longer sequences that are unrelated to their interpretation as ASCII. It excludes such encodings as UTF-7, UTF-16, GSM03.38, and EBCDIC variants.

The term Unicode character is used to mean a Unicode scalar value (i.e. any Unicode code point that is not a surrogate code point). [UNICODE]

2.2 Conformance requirements

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

This specification describes the conformance criteria for user agents (relevant to implementors) and documents (relevant to authors and authoring tool implementors).

There is no implied relationship between document conformance requirements and implementation conformance requirements. User agents are not free to handle non-conformant documents as they please; the processing model described in this specification applies to implementations regardless of the conformity of the input documents.

User agents fall into several (overlapping) categories with different conformance requirements.

Web browsers and other interactive user agents

Web browsers that support the XHTML syntax must process elements and attributes from the HTML namespace found in XML documents as described in this specification, so that users can interact with them, unless the semantics of those elements have been overridden by other specifications.

A conforming XHTML processor would, upon finding an XHTML script element in an XML document, execute the script contained in that element. However, if the element is found within a transformation expressed in XSLT (assuming the user agent also supports XSLT), then the processor would instead treat the script element as an opaque element that forms part of the transform.

Web browsers that support the HTML syntax must process documents labeled as text/html as described in this specification, so that users can interact with them.

User agents that support scripting must also be conforming implementations of the IDL fragments in this specification, as described in the Web IDL specification. [WEBIDL]

Unless explicitly stated, specifications that override the semantics of HTML elements do not override the requirements on DOM objects representing those elements. For example, the script element in the example above would still implement the HTMLScriptElement interface.

Non-interactive presentation user agents

User agents that process HTML and XHTML documents purely to render non-interactive versions of them must comply to the same conformance criteria as Web browsers, except that they are exempt from requirements regarding user interaction.

Typical examples of non-interactive presentation user agents are printers (static UAs) and overhead displays (dynamic UAs). It is expected that most static non-interactive presentation user agents will also opt to lack scripting support.

A non-interactive but dynamic presentation UA would still execute scripts, allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus" is irrelevant when the user cannot interact with the document, the UA would not need to support any of the focus-related DOM APIs.

User agents with no scripting support

Implementations that do not support scripting (or which have their scripting features disabled entirely) are exempt from supporting the events and DOM interfaces mentioned in this specification. For the parts of this specification that are defined in terms of an events model or in terms of the DOM, such user agents must still act as if events and the DOM were supported.

Scripting can form an integral part of an application. Web browsers that do not support scripting, or that have scripting disabled, might be unable to fully convey the author's intent.

Conformance checkers

Conformance checkers must verify that a document conforms to the applicable conformance criteria described in this specification. Automated conformance checkers are exempt from detecting errors that require interpretation of the author's intent (for example, while a document is non-conforming if the content of a blockquote element is not a quote, conformance checkers running without the input of human judgement do not have to check that blockquote elements only contain quoted material).

Conformance checkers must check that the input document conforms when parsed without a browsing context (meaning that no scripts are run, and that the parser's scripting flag is disabled), and should also check that the input document conforms when parsed with a browsing context in which scripts execute, and that the scripts never cause non-conforming states to occur other than transiently during script execution itself. (This is only a "SHOULD" and not a "MUST" requirement because it has been proven to be impossible. [COMPUTABLE])

The term "HTML5 validator" can be used to refer to a conformance checker that itself conforms to the applicable requirements of this specification.

XML DTDs cannot express all the conformance requirements of this specification. Therefore, a validating XML processor and a DTD cannot constitute a conformance checker. Also, since neither of the two authoring formats defined in this specification are applications of SGML, a validating SGML system cannot constitute a conformance checker either.

To put it another way, there are three types of conformance criteria:

Criteria that can be expressed in a DTD.
Criteria that cannot be expressed by a DTD, but can still be checked by a machine.
Criteria that can only be checked by a human.

A conformance checker must check for the first two. A simple DTD-based validator only checks for the first class of errors and is therefore not a conforming conformance checker according to this specification.

Data mining tools

Applications and tools that process HTML and XHTML documents for reasons other than to either render the documents or check them for conformance should act in accordance with the semantics of the documents that they process.

A tool that generates document outlines but increases the nesting level for each paragraph and does not increase the nesting level for each section would not be conforming.

Authoring tools and markup generators

Authoring tools and markup generators must generate conforming documents. Conformance criteria that apply to authors also apply to authoring tools, where appropriate.

Authoring tools are exempt from the strict requirements of using elements only for their specified purpose, but only to the extent that authoring tools are not yet able to determine author intent.

For example, it is not conforming to use an address element for arbitrary contact information; that element can only be used for marking up contact information for the author of the document or section. However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement.

In terms of conformance checking, an editor has to output documents that conform to the same extent that a conformance checker will verify.

When an authoring tool is used to edit a non-conforming document, it may preserve the conformance errors in sections of the document that were not edited during the editing session (i.e. an editing tool is allowed to round-trip erroneous content). However, an authoring tool must not claim that the output is conformant if errors have been so preserved.

Authoring tools are expected to come in two broad varieties: tools that work from structure or semantic data, and tools that work on a What-You-See-Is-What-You-Get media-specific editing basis (WYSIWYG).

The former is the preferred mechanism for tools that author HTML, since the structure in the source information can be used to make informed choices regarding which HTML elements and attributes are most appropriate.

However, WYSIWYG tools are legitimate. WYSIWYG tools should use elements they know are appropriate, and should not use elements that they do not know to be appropriate. This might in certain extreme cases mean limiting the use of flow elements to just a few elements, like div, b, i, and span and making liberal use of the style attribute.

All authoring tools, whether WYSIWYG or not, should make a best effort attempt at enabling users to create well-structured, semantically rich, media-independent content.

Some conformance requirements are phrased as requirements on elements, attributes, methods or objects. Such requirements fall into two categories: those describing content model restrictions, and those describing implementation behavior. Those in the former category are requirements on documents and authoring tools. Those in the second category are requirements on user agents. Similarly, some conformance requirements are phrased as requirements on authors; such requirements are to be interpreted as conformance requirements on the documents that authors produce. (In other words, this specification does not distinguish between conformance criteria on authors and conformance criteria on documents.)

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML (referred to as the XHTML syntax), and one using a custom format inspired by SGML (referred to as the HTML syntax). Implementations may support only one of these two formats, although supporting both is encouraged.

The language in this specification assumes that the user agent expands all entity references, and therefore does not include entity reference nodes in the DOM. If user agents do include entity reference nodes in the DOM, then user agents must handle them as if they were fully expanded when implementing this specification. For example, if a requirement talks about an element's child text nodes, then any text nodes that are children of an entity reference that is a child of that element would be used as well. Entity references to unknown entities must be treated as if they contained just an empty text node for the purposes of the algorithms defined in this specification.

2.2.1 Dependencies

Status: Last call for comments

This specification relies on several other underlying specifications.

XML

Implementations that support the XHTML syntax must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [XMLNS]

DOM

The Document Object Model (DOM) is a representation — a model — of a document and its content. The DOM is not just an API; the conformance criteria of HTML implementations are defined, in this specification, in terms of operations on the DOM. [DOMCORE]

Implementations must support some version of DOM Core and DOM Events, because this specification is defined in terms of the DOM, and some of the features are defined as extensions to the DOM Core interfaces. [DOMCORE] [DOMEVENTS]

Web IDL

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

Except where otherwise specified, if an IDL attribute that is a floating point number type (float) is assigned an Infinity or Not-a-Number (NaN) value, a NOT_SUPPORTED_ERR exception must be raised.

Except where otherwise specified, if a method with an argument that is a floating point number type (float) is passed an Infinity or Not-a-Number (NaN) value, a NOT_SUPPORTED_ERR exception must be raised.

JavaScript

Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [ECMA262]

The term "JavaScript" is used to refer to ECMA262, rather than the official term ECMAScript, since the term JavaScript is more widely known. Similarly, the MIME type used to refer to JavaScript in this specification is text/javascript, since that is the most commonly used type, despite it being an officially obsoleted type according to RFC 4329. [RFC4329]

Media Queries

Implementations must support some version of the Media Queries language. [MQ]

URIs, IRIs, IDNA

Implementations must support the the semantics of URLs defined in the URI and IRI specifications, as well as the semantics of IDNA domain names defined in the Internationalizing Domain Names in Applications (IDNA) specification. [RFC3986] [RFC3987] [RFC3490]

This specification does not require support of any particular network transport protocols, style sheet language, scripting language, or any of the DOM specifications beyond those described above. However, the language described by this specification is biased towards CSS as the styling language, JavaScript as the scripting language, and HTTP as the network protocol, and several features assume that those languages and protocols are in use.

This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.

2.2.2 Extensibility

Status: Last call for comments. ISSUE-41 (Decentralized-extensibility) blocks progress to Last Call

Vendor-specific proprietary extensions to this specification are strongly discouraged. Documents must not use such extensions, as doing so reduces interoperability and fragments the user base, allowing only users of specific user agents to access the content in question.

If vendor-specific markup extensions are needed, they should be done using XML, with elements or attributes from custom namespaces. If such DOM extensions are needed, the members should be prefixed by vendor-specific strings to prevent clashes with future versions of this specification. Extensions must be defined so that the use of extensions neither contradicts nor causes the non-conformance of functionality defined in the specification.

For example, while strongly discouraged from doing so, an implementation "Foo Browser" could add a new IDL attribute "fooTypeTime" to a control's DOM interface that returned the time it took the user to select the current value of a control (say). On the other hand, defining a new control that appears in a form's elements array would be in violation of the above requirement, as it would violate the definition of elements given in this specification.

When vendor-neutral extensions to this specification are needed, either this specification can be updated accordingly, or an extension specification can be written that overrides the requirements in this specification. When someone applying this specification to their activities decides that they will recognise the requirements of such an extension specification, it becomes an applicable specification for the purposes of conformance requirements in this specification.

User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

2.3 Case-sensitivity and string comparison

Comparing two strings in a case-sensitive manner means comparing them exactly, code point for code point.

Comparing two strings in an ASCII case-insensitive manner means comparing them exactly, code point for code point, except that the characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are considered to also match.

Comparing two strings in a compatibility caseless manner means using the Unicode compatibility caseless match operation to compare the two strings. [UNICODE]

Converting a string to ASCII uppercase means replacing all characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) with the corresponding characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

Converting a string to ASCII lowercase means replacing all characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) with the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z).

A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern's length leaves the two strings as matches of each other.

2.4 Common microsyntaxes

There are various places in HTML that accept particular data types, such as dates or numbers. This section describes what the conformance criteria for content in those formats is, and how to parse them.

Implementors are strongly urged to carefully examine any third-party libraries they might consider using to implement the parsing of syntaxes described below. For example, date libraries are likely to implement error handling behavior that differs from what is required in this specification, since error-handling behavior is often not defined in specifications that describe date syntaxes similar to those used in this specification, and thus implementations tend to vary greatly in how they handle errors.

2.4.1 Common parser idioms

Status: Last call for comments

The space characters, for the purposes of this specification, are U+0020 SPACE, U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), U+000C FORM FEED (FF), and U+000D CARRIAGE RETURN (CR).

The White_Space characters are those that have the Unicode property "White_Space". [UNICODE]

The alphanumeric ASCII characters are those in the ranges U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z, U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z.

Some of the micro-parsers described below follow the pattern of having an input variable that holds the string being parsed, and having a position variable pointing at the next character to parse in input.

For parsers based on this pattern, a step that requires the user agent to collect a sequence of characters means that the following algorithm must be run, with characters being the set of characters that can be collected:

Let input and position be the same variables as those of the same name in the algorithm that invoked these steps.
Let result be the empty string.
While position doesn't point past the end of input and the character at position is one of the characters, append that character to the end of result and advance position to the next character in input.
Return result.

The step skip whitespace means that the user agent must collect a sequence of characters that are space characters. The step skip White_Space characters means that the user agent must collect a sequence of characters that are White_Space characters. In both cases, the collected characters are not used. [UNICODE]

When a user agent is to strip line breaks from a string, the user agent must remove any U+000A LINE FEED (LF) and U+000D CARRIAGE RETURN (CR) characters from that string.

The code-point length of a string is the number of Unicode code points in that string.

2.4.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

2.4.3 Keywords and enumerated attributes

Some attributes are defined as taking one of a finite set of keywords. Such attributes are called enumerated attributes. The keywords are each defined to map to a particular state (several keywords might map to the same state, in which case some of the keywords are synonyms of each other; additionally, some of the keywords can be said to be non-conforming, and are only in the specification for historical reasons). In addition, two default states can be given. The first is the invalid value default, the second is the missing value default.

If an enumerated attribute is specified, the attribute's value must be an ASCII case-insensitive match for one of the given keywords that are not said to be non-conforming, with no leading or trailing whitespace.

When the attribute is specified, if its value is an ASCII case-insensitive match for one of the given keywords then that keyword's state is the state that the attribute represents. If the attribute value matches none of the given keywords, but the attribute has an invalid value default, then the attribute represents that state. Otherwise, if the attribute value matches none of the keywords but there is a missing value default state defined, then that is the state represented by the attribute. Otherwise, there is no default, and invalid values must be ignored.

When the attribute is not specified, if there is a missing value default state defined, then that is the state represented by the (missing) attribute. Otherwise, the absence of the attribute means that there is no state represented.

2.4.4 Numbers

2.4.4.1 Non-negative integers

A string is a valid non-negative integer if it consists of one or more characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9).

A valid non-negative integer represents the number that is represented in base ten by that string of digits.

The rules for parsing non-negative integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either zero, a positive integer, or an error. Leading spaces are ignored. Trailing spaces and any trailing garbage characters are ignored.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Skip whitespace.
If position is past the end of input, return an error.
If the character indicated by position is a U+002B PLUS SIGN character (+), advance position to the next character. (The "+" is ignored, but it is not conforming.)
If position is past the end of input, return an error.
If the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then return an error.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and interpret the resulting sequence as a base-ten integer. Let value be that integer.
Return value.

2.4.4.2 Signed integers

A string is a valid integer if it consists of one or more characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), optionally prefixed with a U+002D HYPHEN-MINUS character (-).

A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number that is represented in base ten by that string of digits. A valid integer with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero.

The rules for parsing integers are similar to the rules for non-negative integers, and are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either an integer or an error. Leading spaces are ignored. Trailing spaces and trailing garbage characters are ignored.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Let sign have the value "positive".
Skip whitespace.
If position is past the end of input, return an error.
If the character indicated by position (the first character) is a U+002D HYPHEN-MINUS character (-):
1. Let sign be "negative".
2. Advance position to the next character.
3. If position is past the end of input, return an error.
Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+):
1. Advance position to the next character. (The "+" is ignored, but it is not conforming.)
2. If position is past the end of input, return an error.
If the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then return an error.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and interpret the resulting sequence as a base-ten integer. Let value be that integer.
If sign is "positive", return value, otherwise return the result of subtracting value from zero.

2.4.4.3 Real numbers

A valid floating point number represents the number obtained by multiplying the significand by ten raised to the power of the exponent, where the significand is the first number, interpreted as base ten (including the decimal point and the number after the decimal point, if any, and interpreting the significand as a negative number if the whole string starts with a U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS character (-) between the E and the number and the number is not zero, or else ignoring a U+002B PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the exponent is treated as zero.

The best representation of the number n as a floating point number is the string obtained from applying the JavaScript operator ToString to n. The JavaScript operator ToString is not uniquely determined. When there are multiple possible strings that could be obtained from the JavaScript operator ToString for a particular value, the user agent must always return the same string for that value (though it may differ from the value used by other user agents).

The rules for parsing floating point number values are as given in the following algorithm. This algorithm must be aborted at the first step that returns something. This algorithm will return either a number or an error. Leading spaces are ignored. Trailing spaces and garbage characters are ignored.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Let value have the value 1.
Let divisor have the value 1.
Let exponent have the value 1.
Skip whitespace.
If position is past the end of input, return an error.
If the character indicated by position is a U+002D HYPHEN-MINUS character (-):
1. Change value and divisor to −1.
2. Advance position to the next character.
3. If position is past the end of input, return an error.
If the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then return an error.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and interpret the resulting sequence as a base-ten integer. Multiply value by that integer.
If position is past the end of input, jump to the step labeled conversion.
If the character indicated by position is a U+002E FULL STOP (.), run these substeps:
1. Advance position to the next character.
2. If position is past the end of input, or if the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then jump to the step labeled conversion.
3. Fraction loop: Multiply divisor by ten.
4. Add the value of the character indicated by position, interpreted as a base-ten digit (0..9) and divided by divisor, to value.
5. Advance position to the next character.
6. If position is past the end of input, then jump to the step labeled conversion.
7. If the character indicated by position is one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), jump back to the step labeled fraction loop in these substeps.
If the character indicated by position is a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), run these substeps:
1. Advance position to the next character.
2. If position is past the end of input, then jump to the step labeled conversion.
3. If the character indicated by position is a U+002D HYPHEN-MINUS character (-):
  1. Change exponent to −1.
  2. Advance position to the next character.
  3. If position is past the end of input, then jump to the step labeled conversion.
  Otherwise, if the character indicated by position is a U+002B PLUS SIGN character (+):
  1. Advance position to the next character.
  2. If position is past the end of input, then jump to the step labeled conversion.
4. If the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then jump to the step labeled conversion.
5. Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and interpret the resulting sequence as a base-ten integer. Multiply exponent by that integer.
6. Multiply value by ten raised to the exponentth power.
Conversion: Let S be the set of finite IEEE 754 single-precision floating point values except −0, but with two special values added: 2¹²⁸ and −2¹²⁸.
Let rounded-value be the number in S that is closest to value, selecting the number with an even significand if there are two equally close values. (The two special values 2¹²⁸ and −2¹²⁸ are considered to have even significands for this purpose.)
If rounded-value is 2¹²⁸ or −2¹²⁸, return an error.
Return rounded-value.

2.4.4.4 Ratios

The algorithms described in this section are used by the progress and meter elements.

A valid denominator punctuation character is one of the characters from the table below. There is a value associated with each denominator punctuation character, as shown in the table below.

Denominator Punctuation Character	Value
U+0025 PERCENT SIGN	%	100
U+066A ARABIC PERCENT SIGN	٪	100
U+FE6A SMALL PERCENT SIGN	﹪	100
U+FF05 FULLWIDTH PERCENT SIGN	％	100
U+2030 PER MILLE SIGN	‰	1000
U+2031 PER TEN THOUSAND SIGN	‱	10000

The steps for finding one or two numbers of a ratio in a string are as given in the following algorithm. This algorithm will return either a number, or a number and a denominator character, or two numbers, or nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
If position is past the end of input, then return nothing and abort these steps.
Find a number in the string according to the algorithm below. If this returns a number, let number1 be that number. Otherwise, if it returned nothing or returned an error condition, return nothing and abort these steps.
Skip White_Space characters.
If the character indicated by position (if any) is a valid denominator punctuation character, let denominator be that character. Otherwise, denominator has no value.
If there are any characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) in input at or after the character indicated by position (if any), but denominator has a value, return nothing and abort these steps.
Otherwise, if denominator has a value, return number1 and denominator and abort these steps.
Find a number in the string again. If this returns a number, let number2 be that number. Otherwise, if it returned nothing or an error condition, return number1 and abort these steps.
Skip White_Space characters.
If the character indicated by position (if any) is a valid denominator punctuation character, return nothing and abort these steps.
If there are any characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) in input at or after the character indicated by position (if any), return nothing and abort these steps.
Otherwise, return number1 and number2.

The algorithm to find a number is as follows. It returns either nothing, a number, or an error condition.

Let input and position be the same variables as those of the same name in the algorithm that invoked these steps.
Collect a sequence of characters that are not U+002E FULL STOP characters (.) and are not characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9).
If position is past the end of input, then return nothing and abort these steps.
Collect a sequence of characters that are either U+002E FULL STOP characters (.) or characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and let that be s.
If the first character or the last character of s is a U+002E FULL STOP character (.), or if s contains more than one U+002E FULL STOP character (.), then return an error condition and abort these steps.
Parse s according to the rules for parsing floating point number values, to obtain number. This step cannot fail (s is guaranteed to be a valid floating point number).
Return number.

2.4.4.5 Percentages and lengths

Status: Last call for comments

The rules for parsing dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than or equal to 1.0, or an error; if a number is returned, then it is further categorized as either a percentage or a length.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Skip whitespace.
If position is past the end of input, return an error.
If the character indicated by position is a U+002B PLUS SIGN character (+), advance position to the next character.
Collect a sequence of characters that are U+0030 DIGIT ZERO (0) characters, and discard them.
If position is past the end of input, return an error.
If the character indicated by position is not one of U+0031 DIGIT ONE (1) to U+0039 DIGIT NINE (9), then return an error.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), and interpret the resulting sequence as a base-ten integer. Let value be that number.
If position is past the end of input, return value as a length.
If the character indicated by position is a U+002E FULL STOP character (.):
1. Advance position to the next character.
2. If position is past the end of input, or if the character indicated by position is not one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then return value as a length.
3. Let divisor have the value 1.
4. Fraction loop: Multiply divisor by ten.
5. Add the value of the character indicated by position, interpreted as a base-ten digit (0..9) and divided by divisor, to value.
6. Advance position to the next character.
7. If position is past the end of input, then return value as a length.
8. If the character indicated by position is one of U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), return to the step labeled fraction loop in these substeps.
If position is past the end of input, return value as a length.
If the character indicated by position is a U+0025 PERCENT SIGN character (%), return value as a percentage.
Return value as a length.

2.4.4.6 Lists of integers

A valid list of integers is a number of valid integers separated by U+002C COMMA characters, with no other characters (e.g. no space characters). In addition, there might be restrictions on the number of integers that can be given, or on the range of values allowed.

The rules for parsing a list of integers are as follows:

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Let numbers be an initially empty list of integers. This list will be the result of this algorithm.
If there is a character in the string input at position position, and it is either a U+0020 SPACE, U+002C COMMA, or U+003B SEMICOLON character, then advance position to the next character in input, or to beyond the end of the string if there are no more characters.
If position points to beyond the end of input, return numbers and abort.
If the character in the string input at position position is a U+0020 SPACE, U+002C COMMA, or U+003B SEMICOLON character, then return to step 4.
Let negated be false.
Let value be 0.
Let started be false. This variable is set to true when the parser sees a number or a U+002D HYPHEN-MINUS character (-).
Let got number be false. This variable is set to true when the parser sees a number.
Let finished be false. This variable is set to true to switch parser into a mode where it ignores characters until the next separator.
Let bogus be false.
Parser: If the character in the string input at position position is:
A U+002D HYPHEN-MINUS character
Follow these substeps:
1. If got number is true, let finished be true.
2. If finished is true, skip to the next step in the overall set of steps.
3. If started is true, let negated be false.
4. Otherwise, if started is false and if bogus is false, let negated be true.
5. Let started be true.
A character in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9)
Follow these substeps:
1. If finished is true, skip to the next step in the overall set of steps.
2. Multiply value by ten.
3. Add the value of the digit, interpreted in base ten, to value.
4. Let started be true.
5. Let got number be true.
A U+0020 SPACE character

A U+002C COMMA character

A U+003B SEMICOLON character
Follow these substeps:
1. If got number is false, return the numbers list and abort. This happens if an entry in the list has no digits, as in "1,2,x,4".
2. If negated is true, then negate value.
3. Append value to the numbers list.
4. Jump to step 4 in the overall set of steps.
A character in the range U+0001 to U+001F, U+0021 to U+002B, U+002D to U+002F, U+003A, U+003C to U+0040, U+005B to U+0060, U+007b to U+007F (i.e. any other non-alphabetic ASCII character)
Follow these substeps:
1. If got number is true, let finished be true.
2. If finished is true, skip to the next step in the overall set of steps.
3. Let negated be false.
Any other character
Follow these substeps:
1. If finished is true, skip to the next step in the overall set of steps.
2. Let negated be false.
3. Let bogus be true.
4. If started is true, then return the numbers list, and abort. (The value in value is not appended to the list first; it is dropped.)
Advance position to the next character in input, or to beyond the end of the string if there are no more characters.
If position points to a character (and not to beyond the end of input), jump to the big Parser step above.
If negated is true, then negate value.
If got number is true, then append value to the numbers list.
Return the numbers list and abort.

2.4.4.7 Lists of dimensions

Status: Last call for comments

The rules for parsing a list of dimensions are as follows. These rules return a list of zero or more pairs consisting of a number and a unit, the unit being one of percentage, relative, and absolute.

Let raw input be the string being parsed.
If the last character in raw input is a U+002C COMMA character (,), then remove that character from raw input.
Split the string raw input on commas. Let raw tokens be the resulting list of tokens.
Let result be an empty list of number/unit pairs.
For each token in raw tokens, run the following substeps:
1. Let input be the token.
2. Let position be a pointer into input, initially pointing at the start of the string.
3. Let value be the number 0.
4. Let unit be absolute.
5. If position is past the end of input, set unit to relative and jump to the last substep.
6. If the character at position is a character in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), interpret the resulting sequence as an integer in base ten, and increment value by that integer.
7. If the character at position is a U+002E FULL STOP character (.), run these substeps:
  1. Collect a sequence of characters consisting of space characters and characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). Let s be the resulting sequence.
  2. Remove all space characters in s.
  3. If s is not the empty string, run these subsubsteps:
    1. Let length be the number of characters in s (after the spaces were removed).
    2. Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10^length.
    3. Increment value by fraction.
8. Skip whitespace.
9. If the character at position is a U+0025 PERCENT SIGN character (%), then set unit to percentage.
  
  Otherwise, if the character at position is a U+002A ASTERISK character (*), then set unit to relative.
10. Add an entry to result consisting of the number given by value and the unit given by unit.
Return the list result.

2.4.5 Dates and times

In the algorithms below, the number of days in month month of year year is: 31 if month is 1, 3, 5, 7, 8, 10, or 12; 30 if month is 4, 6, 9, or 11; 29 if month is 2 and year is a number divisible by 400, or if year is a number divisible by 4 but not by 100; and 28 otherwise. This takes into account leap years in the Gregorian calendar. [GREGORIAN]

The digits in the date and time syntaxes defined in this section must be characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), used to express numbers in base ten.

While the formats described here are intended to be subsets of the corresponding ISO8601 formats, this specification defines parsing rules in much more detail than ISO8601. Implementors are therefore encouraged to carefully examine any date parsing libraries before using them to implement the parsing rules described below; ISO8601 libraries might not parse dates and times in exactly the same manner. [ISO8601]

2.4.5.1 Months

A month consists of a specific proleptic Gregorian date with no time-zone information and no date information beyond a year and a month. [GREGORIAN]

A string is a valid month string representing a year year and month month if it consists of the following components in the given order:

The rules to parse a month string are as follows. This will return either a year and month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Parse a month component to obtain year and month. If this returns nothing, then fail.
If position is not beyond the end of input, then fail.
Return year and month.

The rules to parse a month component, given an input string and a position, are as follows. This will return either a year and a month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.
If year is not a number greater than zero, then fail.
If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the month.
If month is not a number in the range 1 ≤ month ≤ 12, then fail.
Return year and month.

2.4.5.2 Dates

A date consists of a specific proleptic Gregorian date with no time-zone information, consisting of a year, a month, and a day. [GREGORIAN]

A string is a valid date string representing a year year, month month, and day day if it consists of the following components in the given order:

The rules to parse a date string are as follows. This will return either a date, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Parse a date component to obtain year, month, and day. If this returns nothing, then fail.
If position is not beyond the end of input, then fail.
Let date be the date with year year, month month, and day day.
Return date.

The rules to parse a date component, given an input string and a position, are as follows. This will return either a year, a month, and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Parse a month component to obtain year and month. If this returns nothing, then fail.
Let maxday be the number of days in month month of year year.
If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the day.
If day is not a number in the range 1 ≤ day ≤ maxday, then fail.
Return year, month, and day.

2.4.5.3 Times

A time consists of a specific time with no time-zone information, consisting of an hour, a minute, a second, and a fraction of a second.

A string is a valid time string representing an hour hour, a minute minute, and a second second if it consists of the following components in the given order:

The rules to parse a time string are as follows. This will return either a time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.
If position is not beyond the end of input, then fail.
Let time be the time with hour hour, minute minute, and second second.
Return time.

The rules to parse a time component, given an input string and a position, are as follows. This will return either an hour, a minute, and a second, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the hour.
If hour is not a number in the range 0 ≤ hour ≤ 23, then fail.
If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the minute.
If minute is not a number in the range 0 ≤ minute ≤ 59, then fail.
Let second be a string with the value "0".
If position is not beyond the end of input and the character at position is a U+003A COLON, then run these substeps:
1. Advance position to the next character in input.
2. If position is beyond the end of input, or at the last character in input, or if the next two characters in input starting at position are not two characters both in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then fail.
3. Collect a sequence of characters that are either characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) or U+002E FULL STOP characters. If the collected sequence has more than one U+002E FULL STOP characters, or if the last character in the sequence is a U+002E FULL STOP character, then fail. Otherwise, let the collected string be second instead of its previous value.
Interpret second as a base-ten number (possibly with a fractional part). Let second be that number instead of the string version.
If second is not a number in the range 0 ≤ second < 60, then fail.
Return hour, minute, and second.

2.4.5.4 Local dates and times

A local date and time consists of a specific proleptic Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, but expressed without a time zone. [GREGORIAN]

A string is a valid local date and time string representing a date and time if it consists of the following components in the given order:

The rules to parse a local date and time string are as follows. This will return either a date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Parse a date component to obtain year, month, and day. If this returns nothing, then fail.
If position is beyond the end of input or if the character at position is not a U+0054 LATIN CAPITAL LETTER T character (T) then fail. Otherwise, move position forwards one character.
Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.
If position is not beyond the end of input, then fail.
Let date be the date with year year, month month, and day day.
Let time be the time with hour hour, minute minute, and second second.
Return date and time.

2.4.5.5 Global dates and times

A global date and time consists of a specific proleptic Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, expressed with a time-zone offset, consisting of a signed number of hours and minutes. [GREGORIAN]

A string is a valid global date and time string representing a date, time, and a time-zone offset if it consists of the following components in the given order:

This format allows for time-zone offsets from -23:59 to +23:59. In practice, however, the range of offsets of actual time zones is -12:00 to +14:00, and the minutes component of offsets of actual time zones is always either 00, 30, or 45.

The best representation of the global date and time string datetime is the valid global date and time string representing datetime with the last character of the string not being a U+005A LATIN CAPITAL LETTER Z character (Z), even if the time zone is UTC, and with a U+002D HYPHEN-MINUS character (-) representing the sign of the time-zone offset when the time zone is UTC.

The rules to parse a global date and time string are as follows. This will return either a time in UTC, with associated time-zone offset information for round tripping or display purposes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Parse a date component to obtain year, month, and day. If this returns nothing, then fail.
If position is beyond the end of input or if the character at position is not a U+0054 LATIN CAPITAL LETTER T character (T) then fail. Otherwise, move position forwards one character.
Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.
If position is beyond the end of input, then fail.
Parse a time-zone offset component to obtain timezone_hours and timezone_minutes. If this returns nothing, then fail.
If position is not beyond the end of input, then fail.
Let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezone_hours hours and timezone_minutes minutes. That moment in time is a moment in the UTC time zone.
Let timezone be timezone_hours hours and timezone_minutes minutes from UTC.
Return time and timezone.

The rules to parse a time-zone offset component, given an input string and a position, are as follows. This will return either time-zone hours and time-zone minutes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

If the character at position is a U+005A LATIN CAPITAL LETTER Z character (Z), then:
1. Let timezone_hours be 0.
2. Let timezone_minutes be 0.
3. Advance position to the next character in input.
Otherwise, if the character at position is either a U+002B PLUS SIGN (+) or a U+002D HYPHEN-MINUS (-), then:
1. If the character at position is a U+002B PLUS SIGN (+), let sign be "positive". Otherwise, it's a U+002D HYPHEN-MINUS (-); let sign be "negative".
2. Advance position to the next character in input.
3. Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the timezone_hours.
4. If timezone_hours is not a number in the range 0 ≤ timezone_hours ≤ 23, then fail.
5. If sign is "negative", then negate timezone_hours.
6. If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.
7. Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the timezone_minutes.
8. If timezone_minutes is not a number in the range 0 ≤ timezone_minutes ≤ 59, then fail.
9. If sign is "negative", then negate timezone_minutes.
Otherwise, fail.
Return timezone_hours and timezone_minutes.

2.4.5.6 Weeks

A week consists of a week-year number and a week number representing a seven-day period starting on a Monday. Each week-year in this calendaring system has either 52 or 53 such seven-day periods, as defined below. The seven-day period starting on the Gregorian date Monday December 29th 1969 (1969-12-29) is defined as week number 1 in week-year 1970. Consecutive weeks are numbered sequentially. The week before the number 1 week in a week-year is the last week in the previous week-year, and vice versa. [GREGORIAN]

A week-year with a number year has 53 weeks if it corresponds to either a year year in the proleptic Gregorian calendar that has a Thursday as its first day (January 1st), or a year year in the proleptic Gregorian calendar that has a Wednesday as its first day (January 1st) and where year is a number divisible by 400, or a number divisible by 4 but not by 100. All other week-years have 52 weeks.

The week number of the last day of a week-year with 53 weeks is 53; the week number of the last day of a week-year with 52 weeks is 52.

The week-year number of a particular day can be different than the number of the year that contains that day in the proleptic Gregorian calendar. The first week in a week-year y is the week that contains the first Thursday of the Gregorian year y.

A string is a valid week string representing a week-year year and week week if it consists of the following components in the given order:

The rules to parse a week string are as follows. This will return either a week-year number and week number, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.
If year is not a number greater than zero, then fail.
If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.
If position is beyond the end of input or if the character at position is not a U+0057 LATIN CAPITAL LETTER W character (W), then fail. Otherwise, move position forwards one character.
Collect a sequence of characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the week.
Let maxweek be the week number of the last day of year year.
If week is not a number in the range 1 ≤ week ≤ maxweek, then fail.
If position is not beyond the end of input, then fail.
Return the week-year number year and the week number week.

2.4.5.7 Vaguer moments in time

A string is a valid date or time string in content if it consists of zero or more White_Space characters, followed by a valid date or time string, followed by zero or more further White_Space characters.

A string is a valid date string with optional time if it is also one of the following:

The rules to parse a date or time string are as follows. The algorithm is invoked with a flag indicating if the in attribute variant or the in content variant is to be used. The algorithm will return either a date, a time, a global date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
For the in content variant: skip White_Space characters.
Set start position to the same position as position.
Set the date present and time present flags to true.
Parse a date component to obtain year, month, and day. If this fails, then set the date present flag to false.
If date present is true, and position is not beyond the end of input, and the character at position is a U+0054 LATIN CAPITAL LETTER T character (T), then advance position to the next character in input.

Otherwise, if date present is true, and either position is beyond the end of input or the character at position is not a U+0054 LATIN CAPITAL LETTER T character (T), then set time present to false.

Otherwise, if date present is false, set position back to the same position as start position.
If the time present flag is true, then parse a time component to obtain hour, minute, and second. If this returns nothing, then set the time present flag to false.
If both the date present and time present flags are false, then fail.
If the date present and time present flags are both true, but position is beyond the end of input, then fail.
If the date present and time present flags are both true, parse a time-zone offset component to obtain timezone_hours and timezone_minutes. If this returns nothing, then fail.
For the in content variant: skip White_Space characters.
If position is not beyond the end of input, then fail.
If the date present flag is true and the time present flag is false, then let date be the date with year year, month month, and day day, and return date.

Otherwise, if the time present flag is true and the date present flag is false, then let time be the time with hour hour, minute minute, and second second, and return time.

Otherwise, let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezone_hours hours and timezone_minutes minutes, that moment in time being a moment in the UTC time zone; let timezone be timezone_hours hours and timezone_minutes minutes from UTC; and return time and timezone.

2.4.6 Colors

A simple color consists of three 8-bit numbers in the range 0..255, representing the red, green, and blue components of the color respectively, in the sRGB color space. [SRGB]

A string is a valid simple color if it is exactly seven characters long, and the first character is a U+0023 NUMBER SIGN character (#), and the remaining six characters are all in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F, U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F, with the first two digits representing the red component, the middle two digits representing the green component, and the last two digits representing the blue component, in hexadecimal.

A string is a valid lowercase simple color if it is a valid simple color and doesn't use any characters in the range U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F.

The rules for parsing simple color values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple color or an error.

Let input be the string being parsed.
If input is not exactly seven characters long, then return an error.
If the first character in input is not a U+0023 NUMBER SIGN character (#), then return an error.
If the last six characters of input are not all in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F, U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F, then return an error.
Let result be a simple color.
Interpret the second and third characters as a hexadecimal number and let the result be the red component of result.
Interpret the fourth and fifth characters as a hexadecimal number and let the result be the green component of result.
Interpret the sixth and seventh characters as a hexadecimal number and let the result be the blue component of result.
Return result.

The rules for serializing simple color values given a simple color are as given in the following algorithm:

Let result be a string consisting of a single U+0023 NUMBER SIGN character (#).
Convert the red, green, and blue components in turn to two-digit hexadecimal numbers using the digits U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) and U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F, zero-padding if necessary, and append these numbers to result, in the order red, green, blue.
Return result, which will be a valid lowercase simple color.

Some obsolete legacy attributes parse colors in a more complicated manner, using the rules for parsing a legacy color value, which are given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple color or an error.

Let input be the string being parsed.
If input is the empty string, then return an error.
If input is an ASCII case-insensitive match for the string "transparent", then return an error.
If input is an ASCII case-insensitive match for one of the keywords listed in the SVG color keywords or CSS2 System Colors sections of the CSS3 Color specification, then return the simple color corresponding to that keyword. [CSSCOLOR]
If input is four characters long, and the first character in input is a U+0023 NUMBER SIGN character (#), and the last three characters of input are all in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F, and U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F, then run these substeps:
1. Let result be a simple color.
2. Interpret the second character of input as a hexadecimal digit; let the red component of result be the resulting number multiplied by 17.
3. Interpret the third character of input as a hexadecimal digit; let the green component of result be the resulting number multiplied by 17.
4. Interpret the fourth character of input as a hexadecimal digit; let the blue component of result be the resulting number multiplied by 17.
5. Return result.
Replace any characters in input that have a Unicode code point greater than U+FFFF (i.e. any characters that are not in the basic multilingual plane) with the two-character string "00".
If input is longer than 128 characters, truncate input, leaving only the first 128 characters.
If the first character in input is a U+0023 NUMBER SIGN character (#), remove it.
Replace any character in input that is not in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F, and U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F with the character U+0030 DIGIT ZERO (0).
While input's length is zero or not a multiple of three, append a U+0030 DIGIT ZERO (0) character to input.
Split input into three strings of equal length, to obtain three components. Let length be the length of those components (one third the length of input).
If length is greater than 8, then remove the leading length-8 characters in each component, and let length be 8.
While length is greater than two and the first character in each component is a U+0030 DIGIT ZERO (0) character, remove that character and reduce length by one.
If length is still greater than two, truncate each component, leaving only the first two characters in each.
Let result be a simple color.
Interpret the first component as a hexadecimal number; let the red component of result be the resulting number.
Interpret the second component as a hexadecimal number; let the green component of result be the resulting number.
Interpret the third component as a hexadecimal number; let the blue component of result be the resulting number.
Return result.

2.4.7 Space-separated tokens

A set of space-separated tokens is a string containing zero or more words separated by one or more space characters, where words consist of any string of one or more characters, none of which are space characters.

An unordered set of unique space-separated tokens is a set of space-separated tokens where none of the words are duplicated.

An ordered set of unique space-separated tokens is a set of space-separated tokens where none of the words are duplicated but where the order of the tokens is meaningful.

Sets of space-separated tokens sometimes have a defined set of allowed values. When a set of allowed values is defined, the tokens must all be from that list of allowed values; other values are non-conforming. If no such set of allowed values is provided, then all values are conforming.

When a user agent has to split a string on spaces, it must use the following algorithm:

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Let tokens be a list of tokens, initially empty.
Skip whitespace
While position is not past the end of input:
1. Collect a sequence of characters that are not space characters.
2. Add the string collected in the previous step to tokens.
3. Skip whitespace
Return tokens.

When a user agent has to remove a token from a string, it must use the following algorithm:

Let input be the string being modified.
Let token be the token being removed. It will not contain any space characters.
Let output be the output string, initially empty.
Let position be a pointer into input, initially pointing at the start of the string.
If position is beyond the end of input, set the string being modified to output, and abort these steps.
If the character at position is a space character:
1. Append the character at position to the end of output.
2. Advance position so it points at the next character in input.
3. Return to step 5 in the overall set of steps.
Otherwise, the character at position is the first character of a token. Collect a sequence of characters that are not space characters, and let that be s.
If s is exactly equal to token, then:
1. Skip whitespace (in input).
2. Remove any space characters currently at the end of output.
3. If position is not past the end of input, and output is not the empty string, append a single U+0020 SPACE character at the end of output.
Otherwise, append s to the end of output.
Return to step 6 in the overall set of steps.

This causes any occurrences of the token to be removed from the string, and any spaces that were surrounding the token to be collapsed to a single space, except at the start and end of the string, where such spaces are removed.

2.4.8 Comma-separated tokens

A set of comma-separated tokens is a string containing zero or more tokens each separated from the next by a single U+002C COMMA character (,), where tokens consist of any string of zero or more characters, neither beginning nor ending with space characters, nor containing any U+002C COMMA characters (,), and optionally surrounded by space characters.

For instance, the string " a ,b,,d d " consists of four tokens: "a", "b", the empty string, and "d d". Leading and trailing whitespace around each token doesn't count as part of the token, and the empty string can be a token.

Sets of comma-separated tokens sometimes have further restrictions on what consists a valid token. When such restrictions are defined, the tokens must all fit within those restrictions; other values are non-conforming. If no such restrictions are specified, then all values are conforming.

When a user agent has to split a string on commas, it must use the following algorithm:

Let input be the string being parsed.
Let position be a pointer into input, initially pointing at the start of the string.
Let tokens be a list of tokens, initially empty.
Token: If position is past the end of input, jump to the last step.
Collect a sequence of characters that are not U+002C COMMA characters (,). Let s be the resulting sequence (which might be the empty string).
Remove any leading or trailing sequence of space characters from s.
Add s to tokens.
If position is not past the end of input, then the character at position is a U+002C COMMA character (,); advance position past that character.
Jump back to the step labeled token.
Return tokens.

2.4.9 References

A valid hash-name reference to an element of type type is a string consisting of a U+0023 NUMBER SIGN character (#) followed by a string which exactly matches the value of the name attribute of an element in the document with type type.

The rules for parsing a hash-name reference to an element of type type are as follows:

If the string being parsed does not contain a U+0023 NUMBER SIGN character, or if the first such character in the string is the last character in the string, then return null and abort these steps.
Let s be the string from the character immediately after the first U+0023 NUMBER SIGN character in the string being parsed up to the end of that string.
Return the first element of type type that has an id attribute whose value is a case-sensitive match for s or a name attribute whose value is a compatibility caseless match for s.

2.4.10 Media queries

A string is a valid media query if it matches the media_query_list production of the Media Queries specification. [MQ]

A string matches the environment of a view if it is the empty string, a string consisting of only space characters, or is a media query that matches that view's environment according to the definitions given in the Media Queries specification. [MQ]

2.5 URLs

Status: Last call for comments. ISSUE-56 (urls-webarch) blocks progress to Last Call

2.5.1 Terminology

A URL is a valid URL if it is a valid Web address as defined by the Web addresses specification. [WEBADDRESSES]

A URL is an absolute URL if it is an absolute Web address as defined by the Web addresses specification. [WEBADDRESSES]

To parse a URL url into its component parts, the user agent must use the parse a Web address algorithm defined by the Web addresses specification. [WEBADDRESSES]

Parsing a URL results in the following components, again as defined by the Web addresses specification:

<scheme>
<host>
<port>
<hostport>
<path>
<query>
<fragment>
<host-specific>

To resolve a URL to an absolute URL relative to either another absolute URL or an element, the user agent must use the resolve a Web address algorithm defined by the Web addresses specification. [WEBADDRESSES]

The document base URL of a Document object is the absolute URL obtained by running these substeps:

Let fallback base url be the document's address.
If fallback base url is about:blank, and the Document's browsing context has a creator browsing context, then let fallback base url be the document base URL of the creator Document instead.
If there is no base element that is both a child of the head element and has an href attribute, then the document base URL is fallback base url.
Otherwise, let url be the value of the href attribute of the first such element.
Resolve url relative to fallback base url (thus, the base href attribute isn't affected by xml:base attributes).
The document base URL is the result of the previous step if it was successful; otherwise it is fallback base url.

This specification defines the URL about:legacy-compat as a reserved, though unresolvable, about: URI. [ABOUT]

The term "URL" in this specification is used in a manner distinct from the precise technical meaning it is given in RFC 3986. Readers familiar with that RFC will find it easier to read this specification if they pretend the term "URL" as used herein is really called something else altogether. This is a willful violation of RFC 3986. [RFC3986]

2.5.2 Dynamic changes to base URLs

Status: Last call for comments

When an xml:base attribute changes, the attribute's element, and all descendant elements, are affected by a base URL change.

When a document's document base URL changes, all elements in that document are affected by a base URL change.

When an element is moved from one document to another, if the two documents have different base URLs, then that element and all its descendants are affected by a base URL change.

When an element is affected by a base URL change, it must act as described in the following list:

If the element is a hyperlink element

If the absolute URL identified by the hyperlink is being shown to the user, or if any data derived from that URL is affecting the display, then the href attribute should be re-resolved relative to the element and the UI updated appropriately.

For example, the CSS :link/:visited pseudo-classes might have been affected.

If the hyperlink has a ping attribute and its absolute URL(s) are being shown to the user, then the ping attribute's tokens should be re-resolved relative to the element and the UI updated appropriately.

If the element is a q, blockquote, section, article, ins, or del element with a cite attribute

If the absolute URL identified by the cite attribute is being shown to the user, or if any data derived from that URL is affecting the display, then the URL should be re-resolved relative to the element and the UI updated appropriately.

Otherwise

The element is not directly affected.

Changing the base URL doesn't affect the image displayed by img elements, although subsequent accesses of the src IDL attribute from script will return a new absolute URL that might no longer correspond to the image being shown.

2.5.3 Interfaces for URL manipulation

An interface that has a complement of URL decomposition IDL attributes will have seven attributes with the following definitions:

The attributes defined to be URL decomposition IDL attributes must act as described for the attributes with the same corresponding names in this section.

In addition, an interface with a complement of URL decomposition IDL attributes will define an input, which is a URL that the attributes act on, and a common setter action, which is a set of steps invoked when any of the attributes' setters are invoked.

The seven URL decomposition IDL attributes have similar requirements.

On getting, if the input is an absolute URL that fulfills the condition given in the "getter condition" column corresponding to the attribute in the table below, the user agent must return the part of the input URL given in the "component" column, with any prefixes specified in the "prefix" column appropriately added to the start of the string and any suffixes specified in the "suffix" column appropriately added to the end of the string. Otherwise, the attribute must return the empty string.

On setting, the new value must first be mutated as described by the "setter preprocessor" column, then mutated by %-escaping any characters in the new value that are not valid in the relevant component as given by the "component" column. Then, if the input is an absolute URL and the resulting new value fulfills the condition given in the "setter condition" column, the user agent must make a new string output by replacing the component of the URL given by the "component" column in the input URL with the new value; otherwise, the user agent must let output be equal to the input. Finally, the user agent must invoke the common setter action with the value of output.

When replacing a component in the URL, if the component is part of an optional group in the URL syntax consisting of a character followed by the component, the component (including its prefix character) must be included even if the new value is the empty string.

The previous paragraph applies in particular to the ":" before a <port> component, the "?" before a <query> component, and the "#" before a <fragment> component.

For the purposes of the above definitions, URLs must be parsed using the URL parsing rules defined in this specification.

Attribute	Component	Getter Condition	Prefix	Suffix	Setter Preprocessor	Setter Condition
`protocol`	<scheme>	—	—	U+003A COLON (:)	Remove all trailing U+003A COLON characters (:)	The new value is not the empty string
`host`	<hostport>	input is hierarchical and uses a server-based naming authority	—	—	—	The new value is not the empty string and input is hierarchical and uses a server-based naming authority
`hostname`	<host>	input is hierarchical and uses a server-based naming authority	—	—	Remove all leading U+002F SOLIDUS characters (/)	The new value is not the empty string and input is hierarchical and uses a server-based naming authority
`port`	<port>	input is hierarchical, uses a server-based naming authority, and contained a <port> component (possibly an empty one)	—	—	Remove any characters in the new value that are not in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9). If the resulting string is empty, set it to a single U+0030 DIGIT ZERO character (0).	input is hierarchical and uses a server-based naming authority
`pathname`	<path>	input is hierarchical	—	—	If it has no leading U+002F SOLIDUS character (/), prepend a U+002F SOLIDUS character (/) to the new value	input is hierarchical
`search`	<query>	input is hierarchical, and contained a <query> component (possibly an empty one)	U+003F QUESTION MARK (?)	—	Remove one leading U+003F QUESTION MARK character (?), if any	input is hierarchical
`hash`	<fragment>	input contained a <fragment> component (possibly an empty one)	U+0023 NUMBER SIGN (#)	—	Remove one leading U+0023 NUMBER SIGN character (#), if any	—

Input URL	`search` value	Explanation
`http://example.com/`	empty string	No <query> component in input URL.
`http://example.com/?`	`?`	There is a <query> component, but it is empty. The question mark in the resulting value is the prefix.
`http://example.com/?test`	`?test`	The <query> component has the value "`test`".
`http://example.com/?test#`	`?test`	The (empty) <fragment> component is not part of the <query> component.

2.6 Fetching resources

Status: Last call for comments

When a user agent is to fetch a resource, optionally from an origin origin, the following steps must be run:

If the resource is identified by the URL about:blank, then return the empty string and abort these steps.
Generate the address of the resource from which Request-URIs are obtained as required by HTTP for the Referer (sic) header from the document's current address of the appropriate Document as given by the following list. [HTTP]

When navigating

The active document of the source browsing context.

When fetching resources for an element

The element's Document.

When fetching resources in response to a call to an API

The active document of the browsing context of the first script.

Remove any <fragment> component from the generated address of the resource from which Request-URIs are obtained.
Perform the remaining steps asynchronously.
If the resource is identified by an absolute URL, and the resource is to be obtained using an idempotent action (such as an HTTP GET or equivalent), and it is already being downloaded for other reasons (e.g. another invocation of this algorithm), and this request would be identical to the previous one (e.g. same Accept and Origin headers), and the user agent is configured such that it is to reuse the data from the existing download instead of initiating a new one, then use the results of the existing download instead of starting a new one.

Otherwise, at a time convenient to the user and the user agent, download (or otherwise obtain) the resource, applying the semantics of the relevant specifications (e.g. performing an HTTP GET or POST operation, or reading the file from disk, following redirects, dereferencing javascript: URLs, etc).

For the purposes of the Referer (sic) header, use the address of the resource from which Request-URIs are obtained generated in the earlier step.

For the purposes of the Origin header, if the fetching algorithm was explicitly initiated from an origin, then the origin that initiated the HTTP request is origin. Otherwise, this is a request from a "privacy-sensitive" context. [ORIGIN]
If there are cookies to be set, then the user agent must run the following substeps:
1. Wait until ownership of the storage mutex can be taken by this instance of the fetching algorithm.
2. Take ownership of the storage mutex.
3. Update the cookies. [COOKIES]
4. Release the storage mutex so that it is once again free.
When the resource is available, or if there is an error of some description, queue a task that uses the resource as appropriate. If the resource can be processed incrementally, as, for instance, with a progressively interlaced JPEG or an HTML file, additional tasks may be queued to process the data as it is downloaded. The task source for these tasks is the networking task source.

If the user agent can determine the actual length of the resource being fetched for an instance of this algorithm, and if that length is finite, then that length is the file's size. Otherwise, the subject of the algorithm (that is, the resource being fetched) has no known size. (For example, the HTTP Content-Length header might provide this information.)

The user agent must also keep track of the number of bytes downloaded for each instance of this algorithm. This number must exclude any out-of-band metadata, such as HTTP headers.

The application cache processing model introduces some changes to the networking model to handle the returning of cached resources.

The navigation processing model handles redirects itself, overriding the redirection handling that would be done by the fetching algorithm.

Whether the type sniffing rules apply to the fetched resource depends on the algorithm that invokes the rules — they are not always applicable.

2.6.1 Protocol concepts

Status: Last call for comments

User agents can implement a variety of transfer protocols, but this specification mostly defines behavior in terms of HTTP. [HTTP]

The HTTP GET method is equivalent to the default retrieval action of the protocol. For example, RETR in FTP. Such actions are idempotent and safe, in HTTP terms.

The HTTP response codes are equivalent to statuses in other protocols that have the same basic meanings. For example, a "file not found" error is equivalent to a 404 code, a server error is equivalent to a 5xx code, and so on.

The HTTP headers are equivalent to fields in other protocols that have the same basic meaning. For example, the HTTP authentication headers are equivalent to the authentication aspects of the FTP protocol.

Status: Last call for comments

Anything in this specification that refers to HTTP also applies to HTTP-over-TLS, as represented by URLs representing the https scheme.

User agents should report certificate errors to the user and must either refuse to download resources sent with erroneous certificates or must act as if such resources were in fact served with no encryption.

User agents should warn the user that there is a potential problem whenever the user visits a page that the user has previously visited, if the page uses less secure encryption on the second visit.

Not doing so can result in users not noticing man-in-the-middle attacks.

If a user connects to a server with a self-signed certificate, the user agent could allow the connection but just act as if there had been no encryption. If the user agent instead allowed the user to override the problem and then displayed the page as if it was fully and safely encrypted, the user could be easily tricked into accepting man-in-the-middle connections.

If a user connects to a server with full encryption, but the page then refers to an external resource that has an expired certificate, then the user agent will act as if the resource was unavailable, possibly also reporting the problem to the user. If the user agent instead allowed the resource to be used, then an attacker could just look for "secure" sites that used resources from a different host and only apply man-in-the-middle attacks to that host, for example taking over scripts in the page.

If a user bookmarks a site that uses a CA-signed certificate, and then later revisits that site directly but the site has started using a self-signed certificate, the user agent could warn the user that a man-in-the-middle attack is likely underway, instead of simply acting as if the page was not encrypted.

2.6.3 Determining the type of a resource

Status: Last call for comments

The Content-Type metadata of a resource must be obtained and interpreted in a manner consistent with the requirements of the Content-Type Processing Model specification. [MIMESNIFF]

The algorithm for extracting an encoding from a Content-Type, given a string s, is given in the Content-Type Processing Model specification. It either returns an encoding or nothing. [MIMESNIFF]

The sniffed type of a resource must be found in a manner consistent with the requirements given in the Content-Type Processing Model specification for finding that sniffed type. [MIMESNIFF]

The rules for sniffing images specifically and the rules for distingushing if a resource is text or binary are also defined in the Content-Type Processing Model specification. Both sets of rules return a MIME type as their result. [MIMESNIFF]

It is imperative that the rules in the Content-Type Processing Model specification be followed exactly. When a user agent uses different heuristics for content type detection than the server expects, security problems can occur. For more details, see the Content-Type Processing Model specification. [MIMESNIFF]

2.7 Common DOM interfaces

2.7.1 Reflecting content attributes in IDL attributes

Some IDL attributes are defined to reflect a particular content attribute. This means that on getting, the IDL attribute returns the current value of the content attribute, and on setting, the IDL attribute changes the value of the content attribute to the given value.

In general, on getting, if the content attribute is not present, the IDL attribute must act as if the content attribute's value is the empty string; and on setting, if the content attribute is not present, it must first be added.

If a reflecting IDL attribute is a DOMString attribute whose content attribute is defined to contain a URL, then on getting, the IDL attribute must resolve the value of the content attribute relative to the element and return the resulting absolute URL if that was successful, or the empty string otherwise; and on setting, must set the content attribute to the specified literal value. If the content attribute is absent, the IDL attribute must return the default value, if the content attribute has one, or else the empty string.

If a reflecting IDL attribute is a DOMString attribute whose content attribute is defined to contain one or more URLs, then on getting, the IDL attribute must split the content attribute on spaces and return the concatenation of resolving each token URL to an absolute URL relative to the element, with a single U+0020 SPACE character between each URL, ignoring any tokens that did not resolve successfully. If the content attribute is absent, the IDL attribute must return the default value, if the content attribute has one, or else the empty string. On setting, the IDL attribute must set the content attribute to the specified literal value.

If a reflecting IDL attribute is a DOMString whose content attribute is an enumerated attribute, and the IDL attribute is limited to only known values, then, on getting, the IDL attribute must return the conforming value associated with the state the attribute is in (in its canonical case), or the empty string if the attribute is in a state that has no associated keyword value; and on setting, if the new value is an ASCII case-insensitive match for one of the keywords given for that attribute, then the content attribute must be set to the conforming value associated with the state that the attribute would be in if set to the given new value, otherwise, if the new value is the empty string, then the content attribute must be removed, otherwise, the setter must raise a SYNTAX_ERR exception.

If a reflecting IDL attribute is a DOMString but doesn't fall into any of the above categories, then the getting and setting must be done in a transparent, case-preserving manner.

If a reflecting IDL attribute is a boolean attribute, then on getting the IDL attribute must return true if the attribute is set, and false if it is absent. On setting, the content attribute must be removed if the IDL attribute is set to false, and must be set to have the same value as its name if the IDL attribute is set to true. (This corresponds to the rules for boolean content attributes.)

If a reflecting IDL attribute is a signed integer type (long) then, on getting, the content attribute must be parsed according to the rules for parsing signed integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, then the default value must be returned instead, or 0 if there is no default value. On setting, the given value must be converted to the shortest possible string representing the number as a valid integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute is a signed integer type (long) that is limited to only non-negative numbers then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or −1 if there is no default value. On setting, if the value is negative, the user agent must fire an INDEX_SIZE_ERR exception. Otherwise, the given value must be converted to the shortest possible string representing the number as a valid non-negative integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute is an unsigned integer type (unsigned long) then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 0 if there is no default value. On setting, the given value must be converted to the shortest possible string representing the number as a valid non-negative integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute is an unsigned integer type (unsigned long) that is limited to only non-negative numbers greater than zero, then the behavior is similar to the previous case, but zero is not allowed. On getting, the content attribute must first be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 1 if there is no default value. On setting, if the value is zero, the user agent must fire an INDEX_SIZE_ERR exception. Otherwise, the given value must be converted to the shortest possible string representing the number as a valid non-negative integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute is a floating point number type (float), then, on getting, the content attribute must be parsed according to the rules for parsing floating point number values, and if that is successful, the resulting value must be returned. If, on the other hand, it fails, or if the attribute is absent, the default value must be returned instead, or 0.0 if there is no default value. On setting, the given value must be converted to the best representation of the number as a floating point number and then that string must be used as the new content attribute value.

The values Infinity and Not-a-Number (NaN) values throw an exception on setting, as defined earlier.

If a reflecting IDL attribute is of the type DOMTokenList or DOMSettableTokenList, then on getting it must return a DOMTokenList or DOMSettableTokenList object (as appropriate) whose underlying string is the element's corresponding content attribute. When the object mutates its underlying string, the content attribute must itself be immediately mutated. When the attribute is absent, then the string represented by the object is the empty string; when the object mutates this empty string, the user agent must add the corresponding content attribute, with its value set to the value it would have been set to after mutating the empty string. The same DOMTokenList or DOMSettableTokenList object must be returned every time for each attribute.

If an element with no attributes has its element.classList.remove() method invoked, the underlying string won't be changed, since the result of removing any token from the empty string is still the empty string. However, if the element.classList.add() method is then invoked, a class attribute will be added to the element with the value of the token to be added.

If a reflecting IDL attribute has the type HTMLElement, or an interface that descends from HTMLElement, then, on getting, it must run the following algorithm (stopping at the first point where a value is returned):

If the corresponding content attribute is absent, then the IDL attribute must return null.
Let candidate be the element that the document.getElementById() method would find if it was passed as its argument the current value of the corresponding content attribute.
If candidate is null, or if it is not type-compatible with the IDL attribute, then the IDL attribute must return null.
Otherwise, it must return candidate.

On setting, if the given element has an id attribute, then the content attribute must be set to the value of that id attribute. Otherwise, the IDL attribute must be set to the empty string.

2.7.2 Collections

When a collection is created, a filter and a root are associated with the collection.

For example, when the HTMLCollection object for the document.images attribute is created, it is associated with a filter that selects only img elements, and rooted at the root of the document.

The collection then represents a live view of the subtree rooted at the collection's root, containing only nodes that match the given filter. The view is linear. In the absence of specific requirements to the contrary, the nodes within the collection must be sorted in tree order.

The rows list is not in tree order.

An attribute that returns a collection must return the same object every time it is retrieved.

2.7.2.1 HTMLCollection

The object's indices of the supported indexed properties are the numbers in the range zero to one less than the number of nodes represented by the collection. If there are no such elements, then there are no supported indexed properties.

The length attribute must return the number of nodes represented by the collection.

The item(index) method must return the indexth node in the collection. If there is no indexth node in the collection, then the method must return null.

The names of the supported named properties consist of the values of the name attributes of each a, applet, area, embed, form, frame, frameset, iframe, img, and object element represented by the collection with a name attribute, plus the list of IDs that the elements represented by the collection have.

The namedItem(key) method must return the first node in the collection that matches the following requirements:

It is an a, applet, area, embed, form, frame, frameset, iframe, img, or object element with a name attribute equal to key, or,
It is an element with an ID equal to key.

If no such elements are found, then the method must return null.

2.7.2.2 HTMLAllCollection

The object's indices of the supported indexed properties and names of the supported named properties are as defined for HTMLCollection objects.

The namedItem(key) method must act according to the following algorithm:

Let collection be an HTMLAllCollection object rooted at the same node as the HTMLAllCollection object on which the method was invoked, whose filter matches only only elements that already match the filter of the HTMLAllCollection object on which the method was invoked and that are either:
- a, applet, area, embed, form, frame, frameset, iframe, img, or object elements with a name attribute equal to key, or,
- elements with an ID equal to key.
If, at the time the method is called, there is exactly one node in collection, then return that node and stop the algorithm.
Otherwise, if, at the time the method is called, collection is empty, return null and stop the algorithm.
Otherwise, return collection.

The tags(tagName) method must return an HTMLAllCollection rooted at the same node as the HTMLAllCollection object on which the method was invoked, whose filter matches only HTML elements whose local name is the tagName argument and that already match the filter of the HTMLAllCollection object on which the method was invoked. In HTML documents, the argument must first be converted to ASCII lowercase.

2.7.2.3 HTMLFormControlsCollection

The object's indices of the supported indexed properties are as defined for HTMLCollection objects.

The names of the supported named properties consist of the values of all the id and name attributes of all the elements represented by the collection.

The namedItem(name) method must act according to the following algorithm:

If, at the time the method is called, there is exactly one node in the collection that has either an id attribute or a name attribute equal to name, then return that node and stop the algorithm.
Otherwise, if there are no nodes in the collection that have either an id attribute or a name attribute equal to name, then return null and stop the algorithm.
Otherwise, create a RadioNodeList object representing a live view of the HTMLFormControlsCollection object, further filtered so that the only nodes in the RadioNodeList object are those that have either an id attribute or a name attribute equal to name. The nodes in the RadioNodeList object must be sorted in tree order.
Return that RadioNodeList object.

Members of the RadioNodeList interface inherited from the NodeList interface must behave as they would on a NodeList object.

The value IDL attribute on the RadioNodeList object, on getting, must return the value returned by running the following steps:

Let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.
If element is null, or if it is an element with no value attribute, return the empty string.
Otherwise, return the value of element's value attribute.

On setting, the value IDL attribute must run the following steps:

Let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose value content attribute is present and equal to the new value, if any. Otherwise, let it be null.
If element is not null, then set its checkedness to true.

2.7.2.4 HTMLOptionsCollection

The HTMLOptionsCollection interface represents a list of option elements. It is always rooted on a select element and has attributes and methods that manipulate that element's descendants.

The object's indices of the supported indexed properties are as defined for HTMLCollection objects.

On getting, the length attribute must return the number of nodes represented by the collection.

On setting, the behavior depends on whether the new value is equal to, greater than, or less than the number of nodes represented by the collection at that time. If the number is the same, then setting the attribute must do nothing. If the new value is greater, then n new option elements with no attributes and no child nodes must be appended to the select element on which the HTMLOptionsCollection is rooted, where n is the difference between the two numbers (new value minus old value). If the new value is lower, then the last n nodes in the collection must be removed from their parent nodes, where n is the difference between the two numbers (old value minus new value).

Setting length never removes or adds any optgroup elements, and never adds new children to existing optgroup elements (though it can remove children from them).

The names of the supported named properties consist of the values of all the id and name attributes of all the elements represented by the collection.

The namedItem(name) method must act according to the following algorithm:

If, at the time the method is called, there is exactly one node in the collection that has either an id attribute or a name attribute equal to name, then return that node and stop the algorithm.
Otherwise, if there are no nodes in the collection that have either an id attribute or a name attribute equal to name, then return null and stop the algorithm.
Otherwise, create a NodeList object representing a live view of the HTMLOptionsCollection object, further filtered so that the only nodes in the NodeList object are those that have either an id attribute or a name attribute equal to name. The nodes in the NodeList object must be sorted in tree order.
Return that NodeList object.

The add(element, before) method must act according to the following algorithm:

If element is not an option or optgroup element, then return and abort these steps.
If element is an ancestor of the select element on which the HTMLOptionsCollection is rooted, then throw a HIERARCHY_REQUEST_ERR exception.
If before is an element, but that element isn't a descendant of the select element on which the HTMLOptionsCollection is rooted, then throw a NOT_FOUND_ERR exception.
If element and before are the same element, then return and abort these steps.
If before is a node, then let reference be that node. Otherwise, if before is an integer, and there is a beforeth node in the collection, let reference be that node. Otherwise, let reference be null.
If reference is not null, let parent be the parent node of reference. Otherwise, let parent be the select element on which the HTMLOptionsCollection is rooted.
Act as if the DOM Core insertBefore() method was invoked on the parent node, with element as the first argument and reference as the second argument.

The remove(index) method must act according to the following algorithm:

If the number of nodes represented by the collection is zero, abort these steps.
If index is not a number greater than or equal to 0 and less than the number of nodes represented by the collection, let element be the first element in the collection. Otherwise, let element be the indexth element in the collection.
Remove element from its parent node.

2.7.2.5 HTMLPropertiesCollection

The object's indices of the supported indexed properties are as defined for HTMLCollection objects.

The names of the supported named properties consist of the property names of all the elements represented by the collection.

The names attribute must return a live DOMStringList object giving the property names of all the elements represented by the collection, listed in tree order, but with duplicates removed, leaving only the first occurrence of each name. The same object must be returned each time.

The namedItem(name) method must return a PropertyNodeList object representing a live view of the HTMLPropertiesCollection object, further filtered so that the only nodes in the PropertyNodeList object are those that have a property name equal to name. The nodes in the PropertyNodeList object must be sorted in tree order, and the same object must be returned each time a particular name is queried.

Members of the PropertyNodeList interface inherited from the NodeList interface must behave as they would on a NodeList object.

The values IDL attribute on the PropertyNodeList object, on getting, must return a newly constructed array whose values are the values obtained from the itemValue DOM property of each of the elements represented by the object, in tree order.

2.7.3 DOMTokenList

DOMTokenList objects are always case-sensitive, even when the underlying string might ordinarily be treated in a case-insensitive manner.

The length attribute must return the number of tokens that result from splitting the underlying string on spaces. This is the length.

The object's indices of the supported indexed properties are the numbers in the range zero to length-1, unless the length is zero, in which case there are no supported indexed properties.

The item(index) method must split the underlying string on spaces, preserving the order of the tokens as found in the underlying string, and then return the indexth item in this list. If index is equal to or greater than the number of tokens, then the method must return null.

For example, if the string is "a b a c" then there are four tokens: the token with index 0 is "a", the token with index 1 is "b", the token with index 2 is "a", and the token with index 3 is "c".

The contains(token) method must run the following algorithm:

If the token argument is the empty string, then raise a SYNTAX_ERR exception and stop the algorithm.
If the token argument contains any space characters, then raise an INVALID_CHARACTER_ERR exception and stop the algorithm.
Otherwise, split the underlying string on spaces to get the list of tokens in the object's underlying string.
If the token indicated by token is a case-sensitive match for one of the tokens in the object's underlying string then return true and stop this algorithm.
Otherwise, return false.

The add(token) method must run the following algorithm:

If the token argument is the empty string, then raise a SYNTAX_ERR exception and stop the algorithm.
If the token argument contains any space characters, then raise an INVALID_CHARACTER_ERR exception and stop the algorithm.
Otherwise, split the underlying string on spaces to get the list of tokens in the object's underlying string.
If the given token is a case-sensitive match for one of the tokens in the DOMTokenList object's underlying string then stop the algorithm.
Otherwise, if the DOMTokenList object's underlying string is not the empty string and the last character of that string is not a space character, then append a U+0020 SPACE character to the end of that string.
Append the value of token to the end of the DOMTokenList object's underlying string.

The remove(token) method must run the following algorithm:

If the token argument is the empty string, then raise a SYNTAX_ERR exception and stop the algorithm.
If the token argument contains any space characters, then raise an INVALID_CHARACTER_ERR exception and stop the algorithm.
Otherwise, remove the given token from the underlying string.

The toggle(token) method must run the following algorithm:

If the token argument is the empty string, then raise a SYNTAX_ERR exception and stop the algorithm.
If the token argument contains any space characters, then raise an INVALID_CHARACTER_ERR exception and stop the algorithm.
Otherwise, split the underlying string on spaces to get the list of tokens in the object's underlying string.
If the given token is a case-sensitive match for one of the tokens in the DOMTokenList object's underlying string then remove the given token from the underlying string and stop the algorithm, returning false.
Otherwise, if the DOMTokenList object's underlying string is not the empty string and the last character of that string is not a space character, then append a U+0020 SPACE character to the end of that string.
Append the value of token to the end of the DOMTokenList object's underlying string.
Return true.

Objects implementing the DOMTokenList interface must stringify to the object's underlying string representation.

2.7.4 DOMSettableTokenList

The DOMSettableTokenList interface is the same as the DOMTokenList interface, except that it allows the underlying string to be directly changed.

An object implementing the DOMSettableTokenList interface must act as defined for the DOMTokenList interface, except for the value attribute defined here.

The value attribute must return the underlying string on getting, and must replace the underlying string with the new value on setting.

2.7.5 Safe passing of structured data

Status: Last call for comments

When a user agent is required to obtain a structured clone of an object, it must run the following algorithm, which either returns a separate object, or throws an exception.

Let input be the object being cloned.
Let memory be a list of objects, initially empty. (This is used to catch cycles.)
Let output be the object resulting from calling the internal structured cloning algorithm with input and memory.
Return output.

The internal structured cloning algorithm is always called with two arguments, input and memory, and its behavior depends on the type of input, as follows:

If input is the undefined value

Return the undefined value.

If input is the null value

Return the null value.

If input is the false value

Return the false value.

If input is the true value

Return the true value.

If input is a Number object

Return a newly constructed Number object with the same value as input.

If input is a String object

Return a newly constructed String object with the same value as input.

If input is a Date object

Return a newly constructed Date object with the same value as input.

If input is a RegExp object

Return a newly constructed RegExp object with the same pattern and flags as input.

The value of the lastIndex property is not copied.

If input is a ImageData object

Return a newly constructed ImageData object with the same width and height as input, and with a newly constructed CanvasPixelArray for its data attribute, with the same length and pixel values as the input's.

If input is a File object

Return a newly constructed File object corresponding to the same underlying data.

If input is a Blob object

Return a newly constructed Blob object corresponding to the same underlying data.

If input is a FileList object

Return a newly constructed FileList object containing a list of newly constructed File objects corresponding to the same underlying data as those in input, maintaining their relative order.

If input is a host object (e.g. a DOM node)

Return the null value.

If input is an Array object

If input is an Object object

If input is in memory, then throw a NOT_SUPPORTED_ERR exception and abort the overall structured clone algorithm.
Otherwise, let new memory be a list consisting of the items in memory with the addition of input.
Create a new object, output, of the same type as input: either an Array or an Object.
For each enumerable property in input, add a corresponding property to output having the same name, and having a value created from invoking the internal structured cloning algorithm recursively with the value of the property as the "input" argument and new memory as the "memory" argument. The order of the properties in the input and output objects must be the same.

This does not walk the prototype chain.
Return output.

If input is another native object type (e.g. Error)

Return the null value.

2.7.6 DOMStringMap

The DOMStringMap interface represents a set of name-value pairs. It exposes these using the scripting language's native mechanisms for property access.

When a DOMStringMap object is instantiated, it is associated with three algorithms, one for getting the list of name-value pairs, one for setting names to certain values, and one for deleting names.

interface DOMStringMap {
  getter DOMString (in DOMString name);
  setter void (in DOMString name, in DOMString value);
  creator void (in DOMString name, in DOMString value);
  deleter void (in DOMString name);
};

The names of the supported named properties on a DOMStringMap object at any instant are the names of each pair returned from the algorithm for getting the list of name-value pairs at that instant.

When a DOMStringMap object is indexed to retrieve a named property name, the value returned must be the value component of the name-value pair whose name component is name in the list returned by the algorithm for getting the list of name-value pairs.

When a DOMStringMap object is indexed to create or modify a named property name with value value, the algorithm for setting names to certain values must be run, passing name as the name and the result of converting value to a DOMString as the value.

When a DOMStringMap object is indexed to delete a named property named name, the algorithm for deleting names must be run, passing name as the name.

The DOMStringMap interface definition here is only intended for JavaScript environments. Other language bindings will need to define how DOMStringMap is to be implemented for those languages.

2.7.7 DOM feature strings

DOM3 Core defines mechanisms for checking for interface support, and for obtaining implementations of interfaces, using feature strings. [DOMCORE]

Authors are strongly discouraged from using these, as they are notoriously unreliable and imprecise. Authors are encouraged to rely on explicit feature testing or the graceful degradation behavior intrinsic to some of the features in this specification.

For historical reasons, user agents should return the true value when the hasFeature(feature, version) method of the DOMImplementation interface is invoked with feature set to either "HTML" or "XHTML" and version set to either "1.0" or "2.0".

2.7.8 Exceptions

2.7.9 Garbage collection

Status: Last call for comments

There is an implied strong reference from any IDL attribute that returns a pre-existing object to that object.

For example, the document.location attribute means that there is a strong reference from a Document object to its Location object. Similarly, there is always a strong reference from a Document to any descendant nodes, and from any node to its owner Document.

3 Semantics, structure, and APIs of HTML documents

3.1 Documents

Every XML and HTML document in an HTML UA is represented by a Document object. [DOMCORE]

Document objects are assumed to be XML documents unless they are flagged as being HTML documents when they are created. Whether a document is an HTML document or an XML document affects the behavior of certain APIs and the case-sensitivity of some selectors.

3.1.1 Documents in the DOM

All Document objects (in user agents implementing this specification) must also implement the HTMLDocument interface, available using binding-specific methods. (This is the case whether or not the document in question is an HTML document or indeed whether it contains any HTML elements at all.) Document objects must also implement the document-level interface of any other namespaces that the UA supports.

For example, if an HTML implementation also supports SVG, then the Document object implements both HTMLDocument and SVGDocument.

Because the HTMLDocument interface is now obtained using binding-specific casting methods instead of simply being the primary interface of the document object, it is no longer defined as inheriting from Document.

Since the HTMLDocument interface holds methods and attributes related to a number of disparate features, the members of this interface are described in various different sections.

3.1.2 Security

3.1.3 Resource metadata management

The URL attribute must return the document's address.

The referrer attribute must return either the current address of the active document of the source browsing context at the time the navigation was started (that is, the page which navigated the browsing context to the current document), with any <fragment> component removed; or the empty string if there is no such originating page, or if the UA has been configured not to report referrers in this case, or if the navigation was initiated for a hyperlink with a noreferrer keyword.

In the case of HTTP, the referrer IDL attribute will match the Referer (sic) header that was sent when fetching the current page.

Typically user agents are configured to not report referrers in the case where the referrer uses an encrypted protocol and the current page does not (e.g. when navigating from an https: page to an http: page).

The cookie attribute represents the cookies of the resource.

On getting, if the document is not associated with a browsing context then the user agent must raise an INVALID_STATE_ERR exception. Otherwise, if the sandboxed origin browsing context flag was set on the browsing context of the Document when the Document was created, the user agent must raise a SECURITY_ERR exception. Otherwise, if the document's address does not use a server-based naming authority, it must return the empty string. Otherwise, it must first obtain the storage mutex and then return the cookie-string for the document's address for a "non-HTTP" API. [COOKIES]

On setting, if the document is not associated with a browsing context then the user agent must raise an INVALID_STATE_ERR exception. Otherwise, if the sandboxed origin browsing context flag was set on the browsing context of the Document when the Document was created, the user agent must raise a SECURITY_ERR exception. Otherwise, if the document's address does not use a server-based naming authority, it must do nothing. Otherwise, the user agent must obtain the storage mutex and then act as it would when receiving a set-cookie-string for the document's address via a "non-HTTP" API, consisting of the new value. [COOKIES]

Since the cookie attribute is accessible across frames, the path restrictions on cookies are only a tool to help manage which cookies are sent to which parts of the site, and are not in any way a security feature.

The lastModified attribute, on getting, must return the date and time of the Document's source file's last modification, in the user's local time zone, in the following format:

The month component of the date.
A U+002F SOLIDUS character (/).
The day component of the date.
A U+002F SOLIDUS character (/).
The year component of the date.
A U+0020 SPACE character.
The hours component of the time.
A U+003A COLON character (:).
The minutes component of the time.
A U+003A COLON character (:).
The seconds component of the time.

All the numeric components above, other than the year, must be given as two digits in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) representing the number in base ten, zero-padded if necessary. The year must be given as the shortest possible string of four or more digits in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) representing the number in base ten, zero-padded if necessary.

The Document's source file's last modification date and time must be derived from relevant features of the networking protocols used, e.g. from the value of the HTTP Last-Modified header of the document, or from metadata in the file system for local files. If the last modification date and time are not known, the attribute must return the current date and time in the above format.

A Document is always set to one of three modes: no quirks mode, the default; quirks mode, used typically for legacy documents; and limited quirks mode, also known as "almost standards" mode. The mode is only ever changed from the default by the HTML parser, based on the presence, absence, or value of the DOCTYPE string.

The compatMode IDL attribute must return the literal string "CSS1Compat" unless the document has been set to quirks mode by the HTML parser, in which case it must instead return the literal string "BackCompat".

Documents have an associated character encoding. When a Document object is created, the document's character encoding must be initialized to UTF-16. Various algorithms during page loading affect this value, as does the charset setter. [IANACHARSET]

The charset IDL attribute must, on getting, return the preferred MIME name of the document's character encoding. On setting, if the new value is an IANA-registered alias for a character encoding supported by the user agent, the document's character encoding must be set to that character encoding. (Otherwise, nothing happens.)

The characterSet IDL attribute must, on getting, return the preferred MIME name of the document's character encoding.

The defaultCharset IDL attribute must, on getting, return the preferred MIME name of a character encoding, possibly the user's default encoding, or an encoding associated with the user's current geographical location, or any arbitrary encoding name.

Each document has a current document readiness. When a Document object is created, it must have its current document readiness set to the string "loading" if the document is associated with an HTML parser or an XML parser, or to the string "complete" otherwise. Various algorithms during page loading affect this value. When the value is set, the user agent must fire a simple event named readystatechange at the Document object.

A Document is said to have an active parser if it is associated with an HTML parser or an XML parser that has not yet been stopped or aborted.

The readyState IDL attribute must, on getting, return the current document readiness.

3.1.4 DOM tree accessors

The html element of a document is the document's root element, if there is one and it's an html element, or null otherwise.

The head element of a document is the first head element that is a child of the html element, if there is one, or null otherwise.

The head attribute, on getting, must return the head element of the document (a head element or null).

The title element of a document is the first title element in the document (in tree order), if there is one, or null otherwise.

The title attribute must, on getting, run the following algorithm:

If the root element is an svg element in the "http://www.w3.org/2000/svg" namespace, and the user agent supports SVG, then return the value that would have been returned by the IDL attribute of the same name on the SVGDocument interface. [SVG]
Otherwise, let value be a concatenation of the data of all the child text nodes of the title element, in tree order, or the empty string if the title element is null.
Replace any sequence of one or more consecutive space characters in value with a single U+0020 SPACE character.
Remove any leading or trailing space characters in value.
Return value.

On setting, the following algorithm must be run. Mutation events must be fired as appropriate.

If the root element is an svg element in the "http://www.w3.org/2000/svg" namespace, and the user agent supports SVG, then the setter must defer to the setter for the IDL attribute of the same name on the SVGDocument interface (if it is readonly, then this will raise an exception). Stop the algorithm here. [SVG]
If the title element is null and the head element is null, then the attribute must do nothing. Stop the algorithm here.
If the title element is null, then a new title element must be created and appended to the head element. Let element be that element. Otherwise, let element be the title element.
The children of element (if any) must all be removed.
A single Text node whose data is the new value being assigned must be appended to element.

The title attribute on the HTMLDocument interface should shadow the attribute of the same name on the SVGDocument interface when the user agent supports both HTML and SVG. [SVG]

The body element of a document is the first child of the html element that is either a body element or a frameset element. If there is no such element, it is null. If the body element is null, then when the specification requires that events be fired at "the body element", they must instead be fired at the Document object.

The body attribute, on getting, must return the body element of the document (either a body element, a frameset element, or null). On setting, the following algorithm must be run:

If the new value is not a body or frameset element, then raise a HIERARCHY_REQUEST_ERR exception and abort these steps.
Otherwise, if the new value is the same as the body element, do nothing. Abort these steps.
Otherwise, if the body element is not null, then replace that element with the new value in the DOM, as if the root element's replaceChild() method had been called with the new value and the incumbent body element as its two arguments respectively, then abort these steps.
Otherwise, the the body element is null. Append the new value to the root element.

The images attribute must return an HTMLCollection rooted at the Document node, whose filter matches only img elements.

The embeds attribute must return an HTMLCollection rooted at the Document node, whose filter matches only embed elements.

The plugins attribute must return the same object as that returned by the embeds attribute.

The links attribute must return an HTMLCollection rooted at the Document node, whose filter matches only a elements with href attributes and area elements with href attributes.

The forms attribute must return an HTMLCollection rooted at the Document node, whose filter matches only form elements.

The scripts attribute must return an HTMLCollection rooted at the Document node, whose filter matches only script elements.

The getElementsByName(name) method takes a string name, and must return a live NodeList containing all the HTML elements in that document that have a name attribute whose value is equal to the name argument (in a case-sensitive manner), in tree order.

The getElementsByClassName(classNames) method takes a string that contains a set of space-separated tokens representing classes. When called, the method must return a live NodeList object containing all the elements in the document, in tree order, that have all the classes specified in that argument, having obtained the classes by splitting a string on spaces. (Duplicates are ignored.) If there are no tokens specified in the argument, then the method must return an empty NodeList. If the document is in quirks mode, then the comparisons for the classes must be done in an ASCII case-insensitive manner, otherwise, the comparisons must be done in a case-sensitive manner.

The getElementsByClassName(classNames) method on the HTMLElement interface must return a live NodeList with the nodes that the HTMLDocument getElementsByClassName() method would return when passed the same argument(s), excluding any elements that are not descendants of the HTMLElement object on which the method was invoked.

HTML, SVG, and MathML elements define which classes they are in by having an attribute with no namespace with the name class containing a space-separated list of classes to which the element belongs. Other specifications may also allow elements in their namespaces to be labeled as being in specific classes.

The HTMLDocument interface supports named properties. The names of the supported named properties at any moment consist of the values of the name content attributes of all the applet, embed, form, iframe, img, and fallback-free object elements in the Document that have name content attributes, and the values of the id content attributes of all the applet and fallback-free object elements in the Document that have id content attributes, and the values of the id content attributes of all the img elements in the Document that have both name content attributes and id content attributes.

When the HTMLDocument object is indexed for property retrieval using a name name, then the user agent must return the value obtained using the following steps:

Let elements be the list of named elements with the name name in the Document.
There will be at least one such element, by definition.
If elements has only one element, and that element is an iframe element, then return the WindowProxy object of the nested browsing context represented by that iframe element, and abort these steps.
Otherwise, if elements has only one element, return that element and abort these steps.
Otherwise return an HTMLCollection rooted at the Document node, whose filter matches only named elements with the name name.

Named elements with the name name, for the purposes of the above algorithm, are those that are either:

applet, embed, form, iframe, img, or fallback-free object elements that have a name content attribute whose value is name, or
applet or fallback-free object elements that have an id content attribute whose value is name, or
img elements that have an id content attribute whose value is name, and that have a name content attribute present also.

An object element is said to be fallback-free if it has no object or embed descendants.

The dir attribute on the HTMLDocument interface is defined along with the dir content attribute.

3.1.5 Creating documents

XML documents can be created from script using the DOM createDocument() method on the DOMImplementation interface.

The createHTMLDocument(title) method, when invoked, must run the following steps:

3.2 Elements

3.2.1 Semantics

Status: Last call for comments. ISSUE-41 (Decentralized-extensibility) blocks progress to Last Call

Elements, attributes, and attribute values in HTML are defined (by this specification) to have certain meanings (semantics). For example, the ol element represents an ordered list, and the lang attribute represents the language of the content.

Authors must not use elements, attributes, and attribute values for purposes other than their appropriate intended semantic purpose. Authors must not use elements, attributes, and attribute values that are not permitted by this specification or other applicable specifications.

For example, the following document is non-conforming, despite being syntactically correct:

<!DOCTYPE HTML>
<html lang="en-GB">
 <head> <title> Demonstration </title> </head>
 <body>
  <table>
   <tr> <td> My favourite animal is the cat. </td> </tr>
   <tr>
    <td>
     —<a href="http://example.org/~ernest/"><cite>Ernest</cite></a>,
     in an essay from 1992
    </td>
   </tr>
  </table>
 </body>
</html>

...because the data placed in the cells is clearly not tabular data (and the cite element mis-used). A corrected version of this document might be:

<!DOCTYPE HTML>
<html lang="en-GB">
 <head> <title> Demonstration </title> </head>
 <body>
  <blockquote>
   <p> My favourite animal is the cat. </p>
  </blockquote>
  <p>
   —<a href="http://example.org/~ernest/">Ernest</a>,
   in an essay from 1992
  </p>
 </body>
</html>

This next document fragment, intended to represent the heading of a corporate site, is similarly non-conforming because the second line is not intended to be a heading of a subsection, but merely a subheading or subtitle (a subordinate heading for the same section).

<body>
 <h1>ABC Company</h1>
 <h2>Leading the way in widget design since 1432</h2>
 ...

The hgroup element is intended for these kinds of situations:

<body>
 <hgroup>
  <h1>ABC Company</h1>
  <h2>Leading the way in widget design since 1432</h2>
 </hgroup>
 ...

In the next example, there is a non-conforming attribute value ("carpet") and a non-conforming attribute ("texture"), which is not permitted by this specification:

<label>Carpet: <input type="carpet" name="c" texture="deep pile"></label>

Here would be an alternative and correct way to mark this up:

<label>Carpet: <input type="text" class="carpet" name="c" data-texture="deep pile"></label>

Through scripting and using other mechanisms, the values of attributes, text, and indeed the entire structure of the document may change dynamically while a user agent is processing it. The semantics of a document at an instant in time are those represented by the state of the document at that instant in time, and the semantics of a document can therefore change over time. User agents must update their presentation of the document as this occurs.

HTML has a progress element that describes a progress bar. If its "value" attribute is dynamically updated by a script, the UA would update the rendering to show the progress changing.

3.2.2 Elements in the DOM

The nodes representing HTML elements in the DOM must implement, and expose to scripts, the interfaces listed for them in the relevant sections of this specification. This includes HTML elements in XML documents, even when those documents are in another context (e.g. inside an XSLT transform).

Elements in the DOM represent things; that is, they have intrinsic meaning, also known as semantics.

The basic interface, from which all the HTML elements' interfaces inherit, and which must be used by elements that have no additional requirements, is the HTMLElement interface.

The HTMLElement interface holds methods and attributes related to a number of disparate features, and the members of this interface are therefore described in various different sections of this specification.

The HTMLUnknownElement interface must be used for HTML elements that are not defined by this specification (or other applicable specifications).

3.2.3 Global attributes

The following attributes are common to and may be specified on all HTML elements (even those not defined in this specification):

The attributes marked with an asterisk have a different meaning when specified on body elements as those elements expose event handlers of the Window object with the same names.

While these attributes apply to all elements, they are not useful on all elements. For example, only media elements will ever receive a volumechange event fired by the user agent.

Custom data attributes (e.g. data-foldername or data-msgid) can be specified on any HTML element, to store custom data specific to the page.

In HTML documents, elements in the HTML namespace may have an xmlns attribute specified, if, and only if, it has the exact value "http://www.w3.org/1999/xhtml". This does not apply to XML documents.

In HTML, the xmlns attribute has absolutely no effect. It is basically a talisman. It is allowed merely to make migration to and from XHTML mildly easier. When parsed by an HTML parser, the attribute ends up in no namespace, not the "http://www.w3.org/2000/xmlns/" namespace like namespace declaration attributes in XML do.

In XML, an xmlns attribute is part of the namespace declaration mechanism, and an element cannot actually have an xmlns attribute in no namespace specified.

To enable assistive technology products to expose a more fine-grained interface than is otherwise possible with HTML elements and attributes, a set of annotations for assistive technology products can be specified.

3.2.3.1 The id attribute

The id attribute specifies its element's unique identifier (ID). The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.

An element's unique identifier can be used for a variety of purposes, most notably as a way to link to specific parts of a document using fragment identifiers, as a way to target an element when scripting, and as a way to style a specific element from CSS.

If the value is not the empty string, user agents must associate the element with the given value (exactly, including any space characters) for the purposes of ID matching within the element's home subtree (e.g. for selectors in CSS or for the getElementById() method in the DOM).

Identifiers are opaque strings. Particular meanings should not be derived from the value of the id attribute.

This specification doesn't preclude an element having multiple IDs, if other mechanisms (e.g. DOM Core methods) can set an element's ID in a way that doesn't conflict with the id attribute.

The id IDL attribute must reflect the id content attribute.

3.2.3.2 The title attribute

The title attribute represents advisory information for the element, such as would be appropriate for a tooltip. On a link, this could be the title or a description of the target resource; on an image, it could be the image credit or a description of the image; on a paragraph, it could be a footnote or commentary on the text; on a citation, it could be further information about the source; and so forth. The value is text.

If this attribute is omitted from an element, then it implies that the title attribute of the nearest ancestor HTML element with a title attribute set is also relevant to this element. Setting the attribute overrides this, explicitly stating that the advisory information of any ancestors is not relevant to this element. Setting the attribute to the empty string indicates that the element has no advisory information.

If the title attribute's value contains U+000A LINE FEED (LF) characters, the content is split into multiple lines. Each U+000A LINE FEED (LF) character represents a line break.

Some elements, such as link, abbr, and input, define additional semantics for the title attribute beyond the semantics described above.

The title IDL attribute must reflect the title content attribute.

3.2.3.3 The lang and xml:lang attributes

The lang attribute (in no namespace) specifies the primary language for the element's contents and for any of the element's attributes that contain text. Its value must be a valid BCP 47 language code, or the empty string. [BCP47]

If these attributes are omitted from an element, then the language of this element is the same as the language of its parent element, if any. Setting the attribute to the empty string indicates that the primary language is unknown.

Authors must not use the lang attribute in the XML namespace on HTML elements in HTML documents. To ease migration to and from XHTML, authors may specify an attribute in no namespace with no prefix and with the literal localname "xml:lang" on HTML elements in HTML documents, but such attributes must only be specified if a lang attribute in no namespace is also specified, and both attributes must have the same value when compared in an ASCII case-insensitive manner.

The attribute in no namespace with no prefix and with the literal localname "xml:lang" has no effect on language processing.

To determine the language of a node, user agents must look at the nearest ancestor element (including the element itself if the node is an element) that has a lang attribute in the XML namespace set or is an HTML element and has a lang in no namespace attribute set. That attribute specifies the language of the node.

If both the lang attribute in no namespace and the lang attribute in the XML namespace are set on an element, user agents must use the lang attribute in the XML namespace, and the lang attribute in no namespace must be ignored for the purposes of determining the element's language.

If no explicit language is given for any ancestors of the node, including the root element, but there is a document-wide default language set, then that is the language of the node.

If there is no document-wide default language, then language information from a higher-level protocol (such as HTTP), if any, must be used as the final fallback language. In the absence of any language information, and in cases where the higher-level protocol reports multiple languages, the language of the node is unknown (the empty string).

If the resulting value is not a recognized language code, then it must be treated as an unknown language (as if the value was the empty string).

User agents may use the element's language to determine proper processing or rendering (e.g. in the selection of appropriate fonts or pronunciations, or for dictionary selection).

The lang IDL attribute must reflect the lang content attribute in no namespace.

3.2.3.4 The xml:base attribute (XML only)

3.2.3.5 The dir attribute

The dir attribute specifies the element's text directionality. The attribute is an enumerated attribute with the keyword ltr mapping to the state ltr, and the keyword rtl mapping to the state rtl. The attribute has no defaults.

The processing of this attribute is primarily performed by the presentation layer. For example, the rendering section in this specification defines a mapping from this attribute to the CSS 'direction' and 'unicode-bidi' properties, and CSS defines rendering in terms of those properties.

The directionality of an element, which is used in particular by the canvas element's text rendering API, is either 'ltr' or 'rtl'. If the user agent supports CSS and the 'direction' property on this element has a computed value of either 'ltr' or 'rtl', then that is the directionality of the element. Otherwise, if the element is being rendered, then the directionality of the element is the directionality used by the presentation layer, potentially determined from the value of the dir attribute on the element. Otherwise, if the element's dir attribute has the state ltr, the element's directionality is 'ltr' (left-to-right); if the attribute has the state rtl, the element's directionality is 'rtl' (right-to-left); and otherwise, the element's directionality is the same as its parent element, or 'ltr' if there is no parent element.

The dir IDL attribute on an element must reflect the dir content attribute of that element, limited to only known values.

The dir IDL attribute on HTMLDocument objects must reflect the dir content attribute of the html element, if any, limited to only known values. If there is no such element, then the attribute must return the empty string and do nothing on setting.

Authors are strongly encouraged to use the dir attribute to indicate text direction rather than using CSS, since that way their documents will continue to render correctly even in the absence of CSS (e.g. as interpreted by search engines).

3.2.3.6 The class attribute

The attribute, if specified, must have a value that is a set of space-separated tokens representing the various classes that the element belongs to.

The classes that an HTML element has assigned to it consists of all the classes returned when the value of the class attribute is split on spaces. (Duplicates are ignored.)

Assigning classes to an element affects class matching in selectors in CSS, the getElementsByClassName() method in the DOM, and other such features.

There are no additional restrictions on the tokens authors can use in the class attribute, but authors are encouraged to use values that describe the nature of the content, rather than values that describe the desired presentation of the content.

The className and classList IDL attributes must both reflect the class content attribute.

3.2.3.7 The style attribute

All HTML elements may have the style content attribute set. If specified, the attribute must contain only a list of zero or more semicolon-separated (;) CSS declarations. [CSS]

In user agents that support CSS, the attribute's value must be parsed when the attribute is added or has its value changed, with its value treated as the body (the part inside the curly brackets) of a declaration block in a rule whose selector matches just the element on which the attribute is set. All URLs in the value must be resolved relative to the element when the attribute is parsed. For the purposes of the CSS cascade, the attribute must be considered to be a 'style' attribute at the author level.

Documents that use style attributes on any of their elements must still be comprehensible and usable if those attributes were removed.

In particular, using the style attribute to hide and show content, or to convey meaning that is otherwise not included in the document, is non-conforming. (To hide and show content, use the hidden attribute.)

The style IDL attribute must return a CSSStyleDeclaration whose value represents the declarations specified in the attribute, if present. Mutating the CSSStyleDeclaration object must create a style attribute on the element (if there isn't one already) and then change its value to be a value representing the serialized form of the CSSStyleDeclaration object. [CSSOM]

3.2.3.8 Embedding custom non-visible data

A custom data attribute is an attribute in no namespace whose name starts with the string "data-", has at least one character after the hyphen, is XML-compatible, and contains no characters in the range U+0041 to U+005A (LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

All attributes on HTML elements in HTML documents get ASCII-lowercased automatically, so the restriction on ASCII uppercase letters doesn't affect such documents.

Custom data attributes are intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements.

These attributes are not intended for use by software that is independent of the site that uses the attributes.

The dataset IDL attribute provides convenient accessors for all the data-* attributes on an element. On getting, the dataset IDL attribute must return a DOMStringMap object, associated with the following algorithms, which expose these attributes on their element:

The algorithm for getting the list of name-value pairs

Let list be an empty list of name-value pairs.
For each content attribute on the element whose first five characters are the string "data-", add a name-value pair to list whose name is the attribute's name with the first five character removed and whose value is the attribute's value.
Return list.

The algorithm for setting names to certain values

Let name be the concatenation of the string data- and the name passed to the algorithm.
Let value be the value passed to the algorithm.
Set the value of the attribute with the name name, to the value value, replacing any previous value if the attribute already existed. If setAttribute() would have raised an exception when setting an attribute with the name name, then this must raise the same exception.

The algorithm for deleting names

Let name be the concatenation of the string data- and the name passed to the algorithm.
Remove the attribute with the name name, if such an attribute exists. Do nothing otherwise.

Authors should carefully design such extensions so that when the attributes are ignored and any associated CSS dropped, the page is still usable.

User agents must not derive any implementation behavior from these attributes or values. Specifications intended for user agents must not define these attributes to have any meaningful values.

3.2.4 Element definitions

Each element in this specification has a definition that includes the following information:

This is then followed by a description of what the element represents, along with any additional normative conformance criteria that may apply to authors and implementations. Examples are sometimes also included.

3.2.5 Content models

Each element defined in this specification has a content model: a description of the element's expected contents. An HTML element must have contents that match the requirements described in the element's content model.

The space characters are always allowed between elements. User agents represent these characters between elements in the source markup as text nodes in the DOM. Empty text nodes and text nodes consisting of just sequences of those characters are considered inter-element whitespace.

Inter-element whitespace, comment nodes, and processing instruction nodes must be ignored when establishing whether an element's contents match the element's content model or not, and must be ignored when following algorithms that define document and element semantics.

An element A is said to be preceded or followed by a second element B if A and B have the same parent node and there are no other element nodes or text nodes (other than inter-element whitespace) between them.

Authors must not use HTML elements anywhere except where they are explicitly allowed, as defined for each element, or as explicitly required by other specifications. For XML compound documents, these contexts could be inside elements from other namespaces, if those elements are defined as providing the relevant contexts.

3.2.5.1 Kinds of content

Each element in HTML falls into zero or more categories that group elements with similar characteristics together. The following broad categories are used in this specification:

Some elements also fall into other categories, which are defined in other parts of this specification.

In addition, certain elements are categorized as form-associated elements and further subcategorized to define their role in various form-related processing models.

Some elements have unique requirements and do not fit into any particular category.

3.2.5.1.1 Metadata content

Metadata content is content that sets up the presentation or behavior of the rest of the content, or that sets up the relationship of the document with other documents, or that conveys other "out of band" information.

Elements from other namespaces whose semantics are primarily metadata-related (e.g. RDF) are also metadata content.

3.2.5.1.2 Flow content

Most elements that are used in the body of documents and applications are categorized as flow content.

HTML5

A vocabulary and associated APIs for HTML and XHTML

Editor's Draft 24 November 2009

Abstract

Status of this document

Table of contents

1 Introduction

1.1 Background

1.2 Audience

1.3 Scope

1.4 History

1.5 Design notes

1.5.1 Serializability of script execution

1.5.2 Compliance with other specifications

1.6 HTML vs XHTML

1.7 Structure of this specification

1.7.1 How to read this specification

1.7.2 Typographic conventions

1.8 A quick introduction to HTML

1.9 Recommended reading

2 Common infrastructure

2.1 Terminology

2.1.1 Resources

2.1.2 XML

2.1.3 DOM trees

2.1.4 Scripting

2.1.5 Plugins

2.1.6 Character encodings

2.2 Conformance requirements

2.2.1 Dependencies

2.2.2 Extensibility

2.3 Case-sensitivity and string comparison

2.4 Common microsyntaxes

2.4.1 Common parser idioms

2.4.2 Boolean attributes

2.4.3 Keywords and enumerated attributes

2.4.4 Numbers

2.4.4.1 Non-negative integers

2.4.4.2 Signed integers

2.4.4.3 Real numbers

2.4.4.4 Ratios

2.4.4.5 Percentages and lengths

2.4.4.6 Lists of integers

2.4.4.7 Lists of dimensions

2.4.5 Dates and times

2.4.5.1 Months

2.4.5.2 Dates

2.4.5.3 Times

2.4.5.4 Local dates and times

2.4.5.5 Global dates and times

2.4.5.6 Weeks

2.4.5.7 Vaguer moments in time

2.4.6 Colors

2.4.7 Space-separated tokens

2.4.8 Comma-separated tokens

2.4.9 References

2.4.10 Media queries

2.5 URLs

2.5.1 Terminology

2.5.2 Dynamic changes to base URLs

2.5.3 Interfaces for URL manipulation

2.6 Fetching resources

2.6.1 Protocol concepts

2.6.2 Encrypted HTTP and related security concerns

2.6.3 Determining the type of a resource

2.7 Common DOM interfaces

2.7.1 Reflecting content attributes in IDL attributes

2.7.2 Collections

2.7.2.1 HTMLCollection

2.7.2.2 HTMLAllCollection

2.7.2.3 HTMLFormControlsCollection

2.7.2.4 HTMLOptionsCollection

2.7.2.5 HTMLPropertiesCollection

2.7.3 DOMTokenList

2.7.4 DOMSettableTokenList

2.7.5 Safe passing of structured data

2.7.6 DOMStringMap

2.7.7 DOM feature strings

2.7.8 Exceptions

2.7.9 Garbage collection

3.2.3.1 The `id` attribute

3.2.3.2 The `title` attribute

3.2.3.3 The `lang` and `xml:lang` attributes

3.2.3.4 The `xml:base` attribute (XML only)

3.2.3.5 The `dir` attribute

3.2.3.6 The `class` attribute

3.2.3.7 The `style` attribute