Polyglot Markup: A robust profile of the HTML5 vocabulary

Processing Instructions and the XML Declaration

Processing Instructions and the XML Declaration are both forbidden in polyglot markup.

Specifying a Document's Character Encoding

Polyglot markup uses the UTF-8 character encoding, the only character encoding for which both HTML and XML require support. HTML requires UTF-8 to be explicitly declared to avoid fallback to a legacy encoding [[!HTML5]]. For XML, UTF-8 is an encoding default. As such, character encoding MAY be left undeclared in XML with the result that UTF-8 is still supported [[!XML10]].

Polyglot markup declares the UTF-8 character encoding in the following ways, which may be used separately or in combination:

Within the document
- By using the Byte Order Mark (BOM) character (preferred).
- By using <meta charset="UTF-8"/> (the HTML encoding declaration).
Outside the document
- By adding "charset=utf-8" to the MIME/HTTP Content-Type header [[!HTTP11]], as the following examples show in HTML and XML, respectively:
```
					Content-type: text/html; charset=utf-8
				
```
```
					Content-type: application/xhtml+xml; charset=utf-8
				
```

The HTML encoding declaration has no effect in XML. When the HTML encoding declaration is the only encoding declaration, the encoding default from XML makes XML parsers treat content as UTF-8.

The W3C Internationalization (i18n) Group recommends to always include a visible encoding declaration in a document, because it helps developers, testers, or translation production managers to check the encoding of a document visually.

The DOCTYPE

Polyglot markup uses a document type declaration (DOCTYPE) specified by section 8.1.1 of [[!HTML5]]. In addition, the DOCTYPE conforms to the following rules:

The string DOCTYPE is in uppercase letters.
The string SYSTEM, if present, is in uppercase letters.
The string PUBLIC, if present, is in uppercase letters.
A Formal Public Identifier (FPI), if present, is a case-sensitive match of the registered FPI to which it points.
A URI, if present in the document type declaration, is a case-sensitive match of the URI to which it points.
- If the URI is the string about:legacy-compat, polyglot markup includes the string in lowercase letters, as required by HTML5.
- If the URI is an http URL, the URI points to the correct resource, using case-sensitive letters.

The string html SHOULD be in lowercase letters, in order to be both well-formed and valid XML; however, the string MAY be in mixed case or uppercase letters and still be well-formed XML.

Note that using about:legacy-compat in XML may yield unpredictable parsing results, depending on the XML processing pipeline.

Polyglot markup does not use document type declarations for HTML4, HTML3, or HTML2, regardless of whether they contain a URI or not and regardless of their effect in HTML5 parsers, as these document type declarations are not compatible with XHTML.

Namespaces

The following rules apply to namespaces used in polyglot markup.

Element-Level Namespaces

[[!HTML5]] introduces undeclared (native) default namespaces for the root HTML element, html, the root SVG element, svg, and the root MathML element, math. Polyglot markup declares the following default namespaces, when the markup languages are included in the document, to maintain XML-compatibility [[!XML10]]:

<html xmlns="http://www.w3.org/1999/xhtml">
<math xmlns="http://www.w3.org/1998/Math/MathML">
<svg xmlns="http://www.w3.org/2000/svg">

Polyglot markup declares the default namespaces on the root HTML element, html, the root SVG element, svg, and the root MathML element math, and on any HTML elements used as children of SVG or MathML elements. Polyglot markup does not declare any other default or prefixed element namespace, because [[!HTML5]] does not natively support the declaring of any other default or prefixed element namespace.

Attribute-Level Namespaces

[[!HTML5]] introduces undeclared (native) support for attributes in the XLink namespace and with the prefix xlink:. Polyglot markup declares the XLink namespace on the HTML root element (html) or once on the foreign element where it is used (svg or math), to maintain XML-compatibility [[!XML10]].

In polyglot markup, the xlink prefix uses the namespace declaration xmlns:xlink="http://www.w3.org/1999/xlink" before using the xlink prefix for the following attributes:

xlink:actuate
xlink:arcrole
xlink:href
xlink:role
xlink:show
xlink:title
xlink:type

Note that there are other prefixed attributes that can be used beyond xlink:href (such as xml:base). Polyglot markup does not declare these prefixes via xmlns. The prefixes are implicitly declared in XML and are automatically applied to the appropriate attributes in HTML.

The prefixed attributes, such as xml:lang="", are "namespaced" within XHTML, SVG and MathML. Thus, they can be styled via CSS3 namespaces. [[!CSS3NAMESPACE]] However, for the HTML serialization, xml:lang would then not have the xml namespace effect. A style such as the following is valid in XHTML, SVG, and MathML, it does not work in HTML and is therefore not used in polyglot markup.

<style type="text/css">
@namespace xml   "http://www.w3.org/XML/1998/namespace";
*[xml|lang]{background:lime;}
</style>

Elements

Polyglot markup conforms to the following rules regarding elements.

Required Elements

Every polyglot markup document contains an html, head, title, and body element. The html element is the root element. The head and body elements are children of the html element. The title element is a child of the head element. Therefore, the following source code would be the most basic polyglot markup document.

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
  <head>
    <title></title>
  </head>
  <body>
  </body>
</html>

Whenever it uses a tr element, polyglot markup always wraps the tr element inside a tbody, thead, or tfoot element. In HTML, if a group of one or more adjacent tr elements are not explictly wrapped inside a tbody, thead, or tfoot element, the HTML parser creates and wraps a new tbody element around the tr elements. XML parsers do not create the tbody element, thus offering the potential for creating different DOMs.

Correct:

<table>
<tbody>
<tr>...

Incorrect:

<table>
<tr>...

Whenever it uses col elements within a table element, polyglot markup explicitly uses a colgroup element surrounding groups of the col elements. In HTML, if a group of one or more adjacent col elements are not explicitly wrapped inside a colgroup element, the HTML parser creates and wraps a new colgroup element around the col elements. XML parsers do not create the colgroup element, thus offering the potential for creating different DOMs.

Correct:

<table>
<colgroup>
<col>...

Incorrect:

<table>
<col>...

Elements that Cannot Be Used in Polyglot Markup

Polyglot markup does not use the noscript element, because the noscript element cannot be used in XML documents. [[!HTML5]]

Case-Sensitivity

The following guidelines apply to any usage of element names, attribute names, or attribute values in markup, script, or CSS. Polyglot markup uses lower case letters for all ASCII letters. For non-ASCII letters—such as Greek, Cyrillic, or non-ASCII Latin letters—polyglot markup respects case sensitivity as it is called for.

Element Names

Polyglot markup uses the correct case for element names.

Polyglot markup uses lowercase letters for all HTML element names.
Polyglot markup uses lowercase letters for all MathML element names.
Polyglot markup uses lowercase letters for all SVG element names except the following, for which polyglot markup uses mixed case:
- altGlyph
- altGlyphDef
- altGlyphItem
- animateColor
- animateMotion
- animateTransform
- clipPath
- feBlend
- feColorMatrix
- feComponentTransfer
- feComposite
- feConvolveMatrix
- feDiffuseLighting
- feDisplacementMap
- feDistantLight
- feFlood
- feFuncA
- feFuncB
- feFuncG
- feFuncR
- feGaussianBlur
- feImage
- feMerge
- feMergeNode
- feMorphology
- feOffset
- fePointLight
- feSpecularLighting
- feSpotLight
- feTile
- feTurbulence
- foreignObject
- glyphRef
- linearGradient
- radialGradient
- textPath

Attribute Names

Polyglot markup uses the correct case for attribute names.

Polyglot markup uses lowercase letters in attribute names for all HTML elements.
Polyglot markup uses lowercase letters in attribute names for all MathML elements except the lowercase definitionurl, which polyglot markup changes to the mixed case definitionURL.
Polyglot markup uses lowercase letters in attribute names for all SVG elements except the following, for which polyglot markup uses mixed case:
- attributeName
- attributeType
- baseFrequency
- baseProfile
- calcMode
- clipPathUnits
- contentScriptType
- contentStyleType
- diffuseConstant
- edgeMode
- externalResourcesRequired
- filterRes
- filterUnits
- glyphRef
- gradientTransform
- gradientUnits
- kernelMatrix
- kernelUnitLength
- keyPoints
- keySplines
- keyTimes
- lengthAdjust
- limitingConeAngle
- markerHeight
- markerUnits
- markerWidth
- maskContentUnits
- maskUnits
- numOctaves
- pathLength
- patternContentUnits
- patternTransform
- patternUnits
- pointsAtX
- pointsAtY
- pointsAtZ
- preserveAlpha
- preserveAspectRatio
- primitiveUnits
- refX
- refY
- repeatCount
- repeatDur
- requiredExtensions
- requiredFeatures
- specularConstant
- specularExponent
- spreadMethod
- startOffset
- stdDeviation
- stitchTiles
- surfaceScale
- systemLanguage
- tableValues
- targetX
- targetY
- textLength
- viewBox
- viewTarget
- xChannelSelector
- yChannelSelector
- zoomAndPan

Attribute Values

For characters in attribute values, polyglot markup maintains case consistency between markup, DOM APIs, and CSS when these attributes are used on HTML elements.

Polyglot markup maintains case consistency for values on the following attributes, which occur on MIME types, language tags, charsets, booleans, media queries, and keywords. Though not required, an easy way to maintain case-consistency is to use only lower case values for these attributes. Polyglot markup maintains case consistency for these values because, for the purpose of selector matching, attribute values in XML are all treated case sensitively; however, HTML treats the values of these attributes as case insensitive (See 4.14.1 Case-sensitivity, in the HTML5 specification). [[!HTML5]]

accept
accept-charset
charset
checked
defer
dir
direction
disabled
enctype
hreflang
http-equiv
lang
media
method
multiple
readonly
rel (for values that do not contain a colon)
scope
selected
shape
target (keywords only; browsing context names are case-sensitive)
type (on a, link, object, script, or style elements)
type (on input)

Note that other specifications, such as RDFa, may place additional restrictions on the allowed values of certain attributes.

Void Elements

Polyglot markup uses only the elements in the following list as void elements.

area
base
br
col
command
embed
hr
img
input
keygen
link
meta
param
source
track
wbr

Polyglot markup uses the minimized tag syntax for void elements, e.g.  , rather than the alternative syntax  .

Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) polyglot markup does not use the minimized form (e.g. the document uses  and not ).

Note that MathML and SVG elements may be either self-closing or contain content.

Elements with Special Considerations

The following elements or their considerations require exceptions to the general rules for polyglot markup.

HTTP Headers and http-equiv Declarations

Content-Language

The HTTP Content-Language: header warrants special discussion in polyglot markup.

HTTP header: Content-language: ru

Whenever there is an HTTP Content-Language: header (whose value is no more and no less than exactly one language tag), polyglot markup declares both the lang and the xml:lang attributes on the root element. For more information, see Language Attributes.

As a general practice and for the sake of expediency and simplicity, polyglot markup may always include both the xml:lang as well as the lang attributes on the root element.

Content-Type

The HTTP Content-Type: header has no extra rules or restrictions, whereas polyglot markup does not use the http-equiv="Content-Type" declaration on the meta element. For more specific information about using the HTTP Content-Type: header, see Specifying a Document's Character Encoding.

Newlines in `textarea` and `pre` Elements

When polyglot markup uses either a textarea or pre element, the text within the element does not begin with a newline.

Attributes

Polyglot markup surrounds all attribute values with quotation marks. Polyglot markup surrounds attribute values with either single quotation marks or with double quotation marks.

Polyglot markup does not use directly typed newline characters within an attribute.

Within an attribute's value, polyglot markup represents tabs, line feeds, and carriage returns as numeric character references rather than by using literal characters. For example, within an attribute's value, polyglot markup uses 	 for a tab rather than the literal character '\t'. This is because of attribute-value normalization in XML [[!XML10]]. The following example uses numeric character references (escaped characters) for the line feed, tab, and less-than characters within a srcdoc attribute.

<iframe srcdoc="&lt;p>Hello &#x0A; &#x09; world!&lt;/p>" src="demo_iframe_srcdoc.htm"></iframe>

Because of attribute-value normalization in XML [[!XML10]], polyglot markup does not use newline characters within an attribute. Practically speaking, for source code with newlines within attributes, DOMs generated via XML and HTML will be different; however, whitespace differences have no behavioral impact on the page unless:

explicitly examined by JavaScript, rendering the differences of small consequence.
used in attributes whose content is rendered visually, such as the content of @alt.

Note that directly typed newline characters are overtly not allowed in any attribute containing a URI.

Disallowed Attributes

The following attributes are not allowed in polyglot markup. These attributes have effects in documents parsed as XML but do not have effects in documents parsed as text/html. The HTML5 spec therefore defines them as invalid in text/html documents. [[!HTML5]]

xml:space
xml:base

Note that the xml:space and xml:base attributes are allowed on SVG and MathML elements.

Language Attributes

When specifying the language mapping of an element, polyglot markup uses both the lang and xml:lang attributes. Neither attribute is to be used without the other, and polyglot markup maintains identical values for both lang and xml:lang.

Polyglot markup uses the language attributes in the html element to set the default language for the document overtly. Although HTML5 sets the language of the root element via a fallback language mechanism, this mechanism is not required to work in XML.

HTML5 activates the fallback language mechanism whenever the root element lacks language attributes. For the mechanism to actually set a fallback language, however, it has to locate either an http-equiv="Content-Language" declaration on the meta element or an HTTP Content-Language: header, either of whose content value is no more and no less than exactly one language tag. Note that although the mechanism can locate either the meta element or the header, the meta element is considered first. For more information about determining language in HTML5, see the language determination rules. [[!HTML5]].

Attributes with Special Considerations

The following attributes or their considerations require exceptions to the general rules for polyglot markup.

The `id` Attribute

Polyglot markup does not contain any space characters within the value of an id attribute. This is because values for the id attribute may not contain space characters in HTML5. [[!HTML5]]

Attributes Styled Using CSS Namespaces

The prefixed attributes, such as xml:lang="", are "namespaced" within XHTML, SVG and MathML. Thus, they can be styled via CSS3 namespaces. [[!CSS3NAMESPACE]] However, for the HTML serialization, xml:lang would then not have the xml namespace effect. A style such as the following is valid in XHTML, SVG, and MathML, it does not work in HTML and is therefore not used in polyglot markup.

<style type="text/css">
@namespace xml   "http://www.w3.org/XML/1998/namespace";
*[xml|lang]{background:lime;}
</style>

Named Entity References

Polyglot markup uses only the following named entity references:

amp
lt
gt
apos
quot

For entities beyond the previous list, polyglot markup uses character references. For example, polyglot markup uses   instead of  . Note that polyglot markup may use decimal values for escape characters (such as   in the previous example); however, the Character Model for the World Wide Web recommends that content SHOULD use the hexadecimal form of character escapes rather than the decimal form when both are available. [[!CHARMOD]]

Polyglot markup always uses character references for the less than sign (<) and ampersand (&) when they are used as characters, except when those characters appear inside a CDATA section.

Script and Style

Polyglot markup includes script and style commands by linking to external files rather than including them in-line. Polyglot markup does not link to an external stylesheet by using the xml-stylesheet processing instruction. See also Processing Instructions and the XML Declaration.

The following examples show how polyglot markup includes external script and style, respectively:

<script src="external.js"></script>

<link rel="stylesheet" href="external.css"/>

Although document.write() and document.writeln() are valid in an HTML document, neither function may be used in XHTML. Therefore, neither is used in polyglot markup. Instead, use the innerHTML property for both HTML and XHTML. Note that the innerHTML property takes a string. XML parsers parse the string as XML in XHTML. HTML parsers parse the string as HTML in HTML. Because of the difference in parsing, if you send the parser content that does not follow the rules for polyglot markup the results will differ for a DOM create with an XML parser and one created with an HTML parser.

Ambiguous Strings

Except for noted exceptions (such as xml:lang="foo"), polyglot markup does not use ambiguous strings. In polyglot markup, ambiguous strings are those strings that XML interprets differently from HTML and vice-versa. Therefore, for the content of script and style tags, polyglot markup does not use the following strings:

String	Notes
<	XML interprets the less than symbol as the commencement of a tag, comment, or CDATA block, even if the symbol occurs within `script` or `style` tags.
&	XML interprets the ampersand as the commencement of a reference or entity, even if the symbol occurs within `script` or `style` tags. As a consequence, polyglot markup does not contain `script` or `style` elements that contain HTML entities, XML entities, or character references.
]]>	XML interprets this string as the end of a CDATA block.

The following example is polyglot markup because there are no ambiguous strings within the script tag.

<script>document.body.appendChild(document.createElement("div"));</script>

A workaround for using ambiguous strings is to include the properly escaped characters inside the src attribute of style or script tags.

Comments in Polyglot Markup

Polyglot markup does not begin a comment with either ">" or "->".

Example Document

The following example code acts as polyglot markup and validates as either XHTML or as HTML. You can view the page live served as HTML, at http://dev.w3.org/html5/html-xhtml-author-guide/SamplePage.html and the same bytes served as XHTML, at http://dev.w3.org/html5/html-xhtml-author-guide/SamplePage.xhtml.

The example document is served as 'text/html'. Some legacy user agents do not support SVG in when served up as 'text/html' as it is in this example. The example page could also be served as 'application/xhtml+xml' instead, with the file extension .html, maintaining adherence to Polyglot markup and enabling the rendering of the SVG.

<!DOCTYPE html>

<html id="SampleDoc" xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">

  <head>
    <title>A Sample Page Using Polglot Markup</title>
    <meta charset='utf-8' />
	<!-- The link element is self-closing as described in Section 6.4 Void Elements -->
	<!-- Style commands are included by linking to an external file rather than including them in-line, 
	  as described in Section 9. Script and Style -->
	<link type="text/css" rel="stylesheet" href="Sample.css"/>
  </head>

  <body>
<nav><p><strong>NB:</strong> These bytes are available served as <a href="SamplePage.xhtml">XHTML</a> and as <a href="SamplePage.html">HTML</a></p></nav>

    <h1>Sample Page Using Polyglot Markup</h1>
    <p>
      The source code for <a href="#SampleDoc">this document</a> uses <dfn id="sampleDef">polyglot markup</dfn>, 
      a document that is a stream of bytes that parses into identical document trees 
      (with the exception of the xmlns attribute on the root element) when processed as HTML and when processed as XML.
      The source code for this document also contains additional comments about the use of 
      <a href="#sampleDef">polyglot markup</a>.
    </p>

    <h2>Foreign Elements</h2>
    <p>
      The following shapes use SVG elements.
      <a href="#sampleDef">Polyglot markup</a> introduces undeclared (native) default namespaces 
      for the the root SVG element (<code>svg</code>) and respects the mixed-case element names and values 
      when appropriate, as described in the sections 5.1 Element-Level Namespaces, 6.3.1 Element Names, and 6.3.3 Attribute Values.
    </p>

    <!-- <a href="#sampleDef">Polyglot markup</a> declares the xlink: namespace on the <svg> element to maintain XML-compatibility  -->
    <svg width="350" height="250" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
      <g>
        <title>Three SVG shapes</title>
        <desc>
          This SVG image contains an ellipse filled with a gradient that goes from white to blue as it moves outward from the center. 
          A yellow rectangle with a black border overlaps the ellipse in the upper-left quadrant, 
          and a red spiral on a white background overlaps the ellipse in the bottom-right quadrant. 
          The red spiral is also a link to the example code for that SVG shape.
        </desc>
        <defs>
          <!-- Note that "radialGradient" and "myGradient" respect mixed-case values. -->
          <radialGradient id="myGradient" cx="50%" cy="50%" r="50%" fx="50%" fy="50%">
            <stop offset="0%" style="stop-color:rgb(200,200,200); stop-opacity:0"/>
            <stop offset="100%" style="stop-color:rgb(0,0,255); stop-opacity:1"/>
          </radialGradient>
        </defs>
      <ellipse cx="50%" cy="50%" rx="50%" ry="42%" style="fill:url(#myGradient)"/>
      <rect x="0" y="0" width="100" height="100" style="fill: yellow; stroke: black;"/> 
      <a xlink:href="http://www.example.org/foo">
        <!--
          Note that the following attribute contains newlines which will produce a different DOM,
          but will not affect the way in which SVG functions in the least.
        -->
        <path transform="translate(60, -175)"
                 d="M153 334 C153 334 151 334 151 334 C151 339 153 344 156 344 C164 344 171 339 171 334 
                    C171 322 164 314 156 314 C142 314 131 322 131 334 C131 350 142 364 156 364 
                    C175 364 191 350 191 334 C191 311 175 294 156 294 C131 294 111 311 111 334 
                    C111 361 131 384 156 384 C186 384 211 361 211 334 C211 300 186 274 156 274"
                 style="fill:white;stroke:red;stroke-width:2"/>
        </a>
      </g>
    </svg> 		
    <h2>Void Elements</h2>
    <!-- Given an empty instance of an element whose content model is not EMPTY (in this case, an empty paragraph) 
    <a href="#sampleDef">polyglot markup</a> does not use the minimized form, as described in Section 6.4 Void Elements -->
    <p></p>
    <p>
      There is an empty <code>p</code> element before this paragraph. 
      <a href="#sampleDef">Polyglot markup</a> uses <code>&lt;p>&lt;/p></code> and not <code>&lt;p/></code>.
    </p>
    <p>
      <a href="#sampleDef">Polyglot markup</a> treats certain elements as self-closing, 
      void elements, such as the following <code>img</code> element.
    </p>
    <img height="48" width="72" alt="W3C" src="http://www.w3.org/Icons/w3c_home"/>
    <p>
      For more information, see Section 6.4 Void Elements.
    </p>


    <h2>Required Elements</h2>
    <p>
      The following table uses the required <code>tbody</code> element, as described in 
      Section 6.1 Required Elements.
    </p>
    <table>
      <tbody>
        <tr>
          <th>Column One</th>
          <th>Column Two</th>
        </tr>
        <tr>
          <td>Row 1, Column 1</td>
          <td>Row 1, Column 2</td>
        </tr>
        <tr>
          <td>Row 2, Column 1</td>
          <td>Row 2, Column 2</td>
        </tr>
        <tr>
          <td>Row 3, Column 1</td>
          <td>Row 3, Column 2</td>
        </tr>
      </tbody>
    </table>

    <p>
      The following table uses the required <code>colgroup</code> element, as described in 
      Section 6.1 Required Elements.  
    </p>
    <table>
      <colgroup>
        <col style="background-color:silver"/>
        <col style="background-color:gray"/>
        <col style="background-color:yellow"/>
      </colgroup>
      <tbody>
        <tr>
          <th>ISBN</th>
          <th>Title</th>
          <th>Price</th>
        </tr>
        <tr>
          <td>3476896</td>
          <td>My first HTML</td>
          <td>$53</td>
        </tr>
        <tr>
          <td>1234567</td>
          <td>Intermediate Polyglot</td>
          <td>$49</td>
        </tr>
      </tbody>
    </table>

    <h2>Named Entity References</h2>
    <p>
      The paragraph you now read, uses the string <code>&amp;amp;</code> for ampersands (“&amp;”) and uses,
      as described in Section 8. Named Entity References, the string <code>&amp;#xA0;</code> 
      for a non-breaking space between the following two words: <i>“<a href="#sampleDef">polyglot&#xA0;markup</a>”</i>.
    </p>
  </body>
</html>

Introduction

Processing Instructions and the XML Declaration

Specifying a Document's Character Encoding

The DOCTYPE

Namespaces

Element-Level Namespaces

Attribute-Level Namespaces

Elements

Required Elements

Elements that Cannot Be Used in Polyglot Markup

Case-Sensitivity

Element Names

Attribute Names

Attribute Values

Void Elements

Elements with Special Considerations

HTTP Headers and http-equiv Declarations

Content-Language

Content-Type

Newlines in `textarea` and `pre` Elements

Attributes

Disallowed Attributes

Language Attributes

Attributes with Special Considerations

The `id` Attribute

Attributes Styled Using CSS Namespaces

Named Entity References

Script and Style

Ambiguous Strings

Comments in Polyglot Markup

Example Document

Acknowledgements

Introduction

Processing Instructions and the XML Declaration

Specifying a Document's Character Encoding

The DOCTYPE

Namespaces

Element-Level Namespaces

Attribute-Level Namespaces

Elements

Required Elements

Elements that Cannot Be Used in Polyglot Markup

Case-Sensitivity

Element Names

Attribute Names

Attribute Values

Void Elements

Elements with Special Considerations

HTTP Headers and http-equiv Declarations

Content-Language

Content-Type

Newlines in textarea and pre Elements

Attributes

Disallowed Attributes

Language Attributes

Attributes with Special Considerations

The id Attribute

Attributes Styled Using CSS Namespaces

Named Entity References

Script and Style

Ambiguous Strings

Comments in Polyglot Markup

Example Document

Acknowledgements

Newlines in `textarea` and `pre` Elements

The `id` Attribute