The a, area, and link
elements can, in certain situations described in the definitions of
those elements, represent hyperlinks.
The href
attribute on a hyperlink element must have a value that is a
valid URL. This URL is the destination
resource of the hyperlink.
The href attribute on
a and area elements is not required; when
those elements do not have href attributes they do not
represent hyperlinks.
The href attribute on the
link element is required, but whether a
link element represents a hyperlink or not depends on
the value of the rel attribute
of that element.
The target
attribute, if present, must be a valid browsing context name
or keyword. It gives the name of the browsing
context that will be used. User agents use
this name when following hyperlinks.
The ping attribute, if
present, gives the URLs of the resources that are interested in
being notified if the user follows the hyperlink. The value must be
a space separated list of one or more valid URLs. The value is used by the user agent for hyperlink
auditing.
For a and area elements that represent
hyperlinks, the relationship between the document containing the
hyperlink and the destination resource indicated by the hyperlink is
given by the value of the element's rel attribute, which
must be a set of space-separated tokens. The allowed values and their meanings are defined
below. The rel attribute has
no default value. If the attribute is omitted or if none of the
values in the attribute are recognized by the user agent, then the
document has no particular relationship with the destination
resource other than there being a hyperlink between the two.
The media
attribute describes for which media the target document was
designed. It is purely advisory. The value must be a valid media query. [MQ] The default,
if the media attribute is
omitted, is all.
The hreflang
attribute on hyperlink elements, if present, gives the language of
the linked resource. It is purely advisory. The value must be a
valid BCP 47 language code. [BCP47]
User agents must not consider this attribute
authoritative — upon fetching the resource, user agents must
use only language information associated with the resource to
determine its language, not metadata included in the link to the
resource.
The type
attribute, if present, gives the MIME type of the linked
resource. It is purely advisory. The value must be a valid
MIME type, optionally with parameters. User agents must not consider the type attribute authoritative
— upon fetching the resource, user agents must not use
metadata included in the link to the resource to determine its
type.
When a user follows a hyperlink, the user agent must
resolve the URL
given by the href attribute
of that hyperlink, relative to the hyperlink element, and if that is
successful, must navigate a browsing
context to the resulting absolute URL. In the
case of server-side image maps, the URL of the hyperlink must
further have its hyperlink suffix appended to it.
If resolving the URL fails, the user agent may report the error to the user in a user-agent-specific manner, may navigate to an error page to report the error, or may ignore the error and do nothing.
If the user indicated a specific browsing context when following the hyperlink, or if the user agent is configured to follow hyperlinks by navigating a particular browsing context, then that must be the browsing context that is navigated.
Otherwise, if the hyperlink element is an a or
area element that has a target attribute, then the
browsing context that is navigated must be chosen by
applying the rules for choosing a browsing context given a
browsing context name, using the value of the target attribute as the
browsing context name. If these rules result in the creation of a
new browsing context, it must be navigated with
replacement enabled.
Otherwise, if the hyperlink element is a sidebar hyperlink and the user agent implements a feature that can be considered a secondary browsing context, such a secondary browsing context may be selected as the browsing context to be navigated.
Otherwise, if the hyperlink element is an a or
area element with no target attribute, but one of
the child nodes of the head element is a
base element with a target attribute, then the browsing
context that is navigated must be chosen by applying the rules
for choosing a browsing context given a browsing context name,
using the value of the target
attribute of the first such base element as the
browsing context name. If these rules result in the creation of a
new browsing context, it must be navigated with
replacement enabled.
Otherwise, the browsing context that must be navigated is the same browsing context as the one which the hyperlink element itself is in.
The navigation must be done with the browsing
context that contains the Document object with
which the hyperlink's element in question is associated as the
source browsing context.
If an a or area hyperlink element has a
ping attribute, and the
user follows the hyperlink, and the hyperlink's URL can
be resolved, relative to the
hyperlink element, without failure, then the user agent must take
the ping attribute's value,
split that string on
spaces, resolve each
resulting token relative to the hyperlink element, and then should
send a request (as described below) to each of the resulting absolute URLs. (Tokens that fail to
resolve are ignored.) This may be done in parallel with the primary
request, and is independent of the result of that request.
User agents should allow the user to adjust this behavior, for
example in conjunction with a setting that disables the sending of
HTTP Referer (sic) headers. Based
on the user's preferences, UAs may either ignore the
ping attribute altogether,
or selectively ignore URLs in the list (e.g. ignoring any
third-party URLs).
For URLs that are HTTP URLs, the requests must be performed by
fetching the specified URLs using the
POST method, with an entity body with the MIME type
text/ping consisting of the four-character string
"PING". All relevant cookie and HTTP
authentication headers must be included in the request. Which other
headers are required depends on the URLs involved.
Document object containing the hyperlink being
audited and the ping URL have the same originPing-From HTTP header with, as its
value, the address of
the document containing the hyperlink, and a Ping-To HTTP header with, as its value,
the address of the absolute URL of the target of the
hyperlink. The request must not include a Referer (sic) HTTP header. Referer (sic)
HTTP header [sic] with, as its value, the current address of the document
containing the hyperlink, a Ping-From HTTP header with the same
value, and a Ping-To HTTP header
with, as its value, the address of the target of the
hyperlink.Ping-To HTTP header with, as its value,
the address of the target of the hyperlink. The request must
neither include a Referer (sic) HTTP header
nor include a Ping-From HTTP
header.In addition, an Origin header
must always be included, whose value is the ASCII serialization of the
origin of the Document containing the
hyperlink.
To save bandwidth, implementors might also wish to
consider omitting optional headers such as Accept from
these requests.
User agents must, unless otherwise specified by the user, honor the HTTP headers (including, in particular, redirects and HTTP cookie headers), but must ignore any entity bodies returned in the responses. User agents may close the connection prematurely once they start receiving an entity body. [RFC2109] [COOKIES]
For URLs that are not HTTP URLs, the requests must be performed by fetching the specified URL normally, and discarding the results.
When the ping attribute is
present, user agents should clearly indicate to the user that
following the hyperlink will also cause secondary requests to be
sent in the background, possibly including listing the actual target
URLs.
For example, a visual user agent could include the hostnames of the target ping URLs along with the hyperlink's actual URL in a status bar or tooltip.
The ping attribute is redundant
with pre-existing technologies like HTTP redirects and JavaScript
in allowing Web pages to track which off-site links are most
popular or allowing advertisers to track click-through rates.
However, the ping attribute
provides these advantages to the user over those alternatives:
Thus, while it is possible to track users without this feature,
authors are encouraged to use the ping attribute so that the user
agent can make the user experience more transparent.
The following table summarizes the link types that are defined by this specification. This table is non-normative; the actual definitions for the link types are given in the next few sections.
In this section, the term referenced document refers to the resource identified by the element representing the link, and the term current document refers to the resource within which the element representing the link finds itself.
To determine which link types apply to a link,
a, or area element, the element's rel attribute must be split on spaces. The resulting tokens are the link
types that apply to that element.
Unless otherwise specified, a keyword must not be specified more
than once per rel
attribute.
The link types are ASCII case-insensitive values, and must be compared as such.
Thus, rel="next" is the
same as rel="NEXT".
| Link type | Effect on... | Brief description | |
|---|---|---|---|
link |
a and area |
||
alternate |
Hyperlink | Hyperlink | Gives alternate representations of the current document. |
archives |
Hyperlink | Hyperlink | Provides a link to a collection of records, documents, or other materials of historical interest. |
author |
Hyperlink | Hyperlink | Gives a link to the current document's author. |
bookmark |
not allowed | Hyperlink | Gives the permalink for the nearest ancestor section. |
external |
not allowed | Hyperlink | Indicates that the referenced document is not part of the same site as the current document. |
feed |
Hyperlink | Hyperlink | Gives the address of a syndication feed for the current document. |
first |
Hyperlink | Hyperlink | Indicates that the current document is a part of a series, and that the first document in the series is the referenced document. |
help |
Hyperlink | Hyperlink | Provides a link to context-sensitive help. |
icon |
External Resource | not allowed | Imports an icon to represent the current document. |
index |
Hyperlink | Hyperlink | Gives a link to the document that provides a table of contents or index listing the current document. |
last |
Hyperlink | Hyperlink | Indicates that the current document is a part of a series, and that the last document in the series is the referenced document. |
license |
Hyperlink | Hyperlink | Indicates that the main content of the current document is covered by the copyright license described by the referenced document. |
next |
Hyperlink | Hyperlink | Indicates that the current document is a part of a series, and that the next document in the series is the referenced document. |
nofollow |
not allowed | Hyperlink | Indicates that the current document's original author or publisher does not endorse the referenced document. |
noreferrer |
not allowed | Hyperlink | Requires that the user agent not send an HTTP Referer (sic) header if the user follows the hyperlink. |
pingback |
External Resource | not allowed | Gives the address of the pingback server that handles pingbacks to the current document. |
prefetch |
External Resource | not allowed | Specifies that the target resource should be preemptively cached. |
prev |
Hyperlink | Hyperlink | Indicates that the current document is a part of a series, and that the previous document in the series is the referenced document. |
search |
Hyperlink | Hyperlink | Gives a link to a resource that can be used to search through the current document and its related pages. |
stylesheet |
External Resource | not allowed | Imports a stylesheet. |
sidebar |
Hyperlink | Hyperlink | Specifies that the referenced document, if retrieved, is intended to be shown in the browser's sidebar (if it has one). |
tag |
Hyperlink | Hyperlink | Gives a tag (identified by the given address) that applies to the current document. |
up |
Hyperlink | Hyperlink | Provides a link to a document giving the context for the current document. |
Some of the types described below list synonyms for these values. These are to be handled as specified by user agents, but must not be used in documents.
alternate"The alternate keyword may be
used with link, a, and area
elements. For link elements, if the rel attribute does not also contain the
keyword stylesheet, it creates a
hyperlink; but if it
does also contain the keyword stylesheet, the alternate keyword instead modifies the
meaning of the stylesheet
keyword in the way described for that keyword, and the rest of this
subsection doesn't apply.
The alternate keyword
indicates that the referenced document is an alternate
representation of the current document.
The nature of the referenced document is given by the media, hreflang, and type attributes.
If the alternate keyword is
used with the media
attribute, it indicates that the referenced document is intended for
use with the media specified.
If the alternate keyword is
used with the hreflang
attribute, and that attribute's value differs from the root
element's language, it indicates that the
referenced document is a translation.
If the alternate keyword is
used with the type
attribute, it indicates that the referenced document is a
reformulation of the current document in the specified format.
The media, hreflang, and type attributes can be combined
when specified with the alternate
keyword.
For example, the following link is a French translation that uses the PDF format:
<link rel=alternate type=application/pdf hreflang=fr href=manual-fr>
If the alternate keyword is
used with the type
attribute set to the value application/rss+xml
or the value application/atom+xml, then the
user agent must treat the link as it would if it had the feed keyword specified as well.
The alternate link
relationship is transitive — that is, if a document links to
two other documents with the link type "alternate", then, in addition to
implying that those documents are alternative representations of the
first document, it is also implying that those two documents are
alternative representations of each other.
archives"The archives keyword may be
used with link, a, and area
elements. For link elements, it creates a hyperlink.
The archives keyword indicates
that the referenced document describes a collection of records,
documents, or other materials of historical interest.
A blog's index page could link to an index of the
blog's past posts with rel="archives".
Synonyms: For historical reasons, user agents
must also treat the keyword "archive" like the
archives keyword.
author"The author keyword may be
used with link, a, and area
elements. For link elements, it creates a hyperlink.
For a and area elements, the author keyword indicates that the
referenced document provides further information about the author of
the nearest article element ancestor of the element
defining the hyperlink, if there is one, or of the page as a whole,
otherwise.
For link elements, the author keyword indicates that the
referenced document provides further information about the author
for the page as a whole.
The "referenced document" can be, and often is, a
mailto: URL giving the e-mail address of the
author. [MAILTO]
Synonyms: For historical reasons, user agents
must also treat link, a, and
area elements that have a rev
attribute with the value "made" as having the author keyword specified as a link
relationship.
bookmark"The bookmark keyword may be
used with a and area elements.
The bookmark keyword gives a
permalink for the nearest ancestor article element of
the linking element in question, or of the section the linking element is most
closely associated with, if there are no ancestor
article elements.
The following snippet has three permalinks. A user agent could determine which permalink applies to which part of the spec by looking at where the permalinks are given.
...
<body>
<h1>Example of permalinks</h1>
<div id="a">
<h2>First example</h2>
<p><a href="a.html" rel="bookmark">This</a> permalink applies to
only the content from the first H2 to the second H2. The DIV isn't
exactly that section, but it roughly corresponds to it.</p>
</div>
<h2>Second example</h2>
<article id="b">
<p><a href="b.html" rel="bookmark">This</a> permalink applies to
the outer ARTICLE element (which could be, e.g., a blog post).</p>
<article id="c">
<p><a href="c.html" rel="bookmark">This</a> permalink applies to
the inner ARTICLE element (which could be, e.g., a blog comment).</p>
</article>
</article>
</body>
...
external"The external keyword may be
used with a and area elements.
The external keyword indicates
that the link is leading to a document that is not part of the site
that the current document forms a part of.
feed"The feed keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The feed keyword indicates that the
referenced document is a syndication feed. If the alternate link type is also specified,
then the feed is specifically the feed for the current document;
otherwise, the feed is just a syndication feed, not necessarily
associated with a particular Web page.
The first link, a, or area
element in the document (in tree order) that creates a hyperlink
with the link type feed must be
treated as the default syndication feed for the purposes of feed
autodiscovery.
The feed keyword is
implied by the alternate link
type in certain cases (q.v.).
The following two link elements are equivalent:
both give the syndication feed for the current page:
<link rel="alternate" type="application/atom+xml" href="data.xml">
<link rel="feed alternate" href="data.xml">
The following extract offers various different syndication feeds:
<p>You can access the planets database using Atom feeds:</p> <ul> <li><a href="recently-visited-planets.xml" rel="feed">Recently Visited Planets</a></li> <li><a href="known-bad-planets.xml" rel="feed">Known Bad Planets</a></li> <li><a href="unexplored-planets.xml" rel="feed">Unexplored Planets</a></li> </ul>
help"The help keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
For a and area elements, the help keyword indicates that the referenced
document provides further help information for the parent of the
element defining the hyperlink, and its children.
In the following example, the form control has associated context-sensitive help. The user agent could use this information, for example, displaying the referenced document if the user presses the "Help" or "F1" key.
<p><label> Topic: <input name=topic> <a href="help/topic.html" rel="help">(Help)</a></label></p>
For link elements, the help keyword indicates that the referenced
document provides help for the page as a whole.
icon"The icon keyword may be used with
link elements, for which it creates an external resource link.
The specified resource is an icon representing the page or site, and should be used by the user agent when representing the page in the user interface.
Icons could be auditory icons, visual icons, or other kinds of
icons. If multiple icons are provided, the user
agent must select the most appropriate icon according to the type, media, and sizes attributes. If there are
multiple equally appropriate icons, user agents must use the last
one declared in tree order. If the user agent tries to
use an icon but that icon is determined, upon closer examination, to
in fact be inappropriate (e.g. because it uses an unsupported
format), then the user agent must try the next-most-appropriate icon
as determined by the attributes.
There is no default type for resources given by the icon keyword. However, for the purposes of
determining the type of the
resource, user agents must expect the resource to be an image.
The sizes
attribute gives the sizes of icons for visual media.
If specified, the attribute must have a value that is an
unordered set of unique space-separated tokens. The
values must all be either any or a value that consists of
two valid non-negative
integers that do not have a leading U+0030 DIGIT ZERO (0)
character and that are separated by a single U+0078 LATIN SMALL
LETTER X character.
The keywords represent icon sizes.
To parse and process the attribute's value, the user agent must first split the attribute's value on spaces, and must then parse each resulting keyword to determine what it represents.
The any keyword
represents that the resource contains a scalable icon, e.g. as
provided by an SVG image.
Other keywords must be further parsed as follows to determine what they represent:
If the keyword doesn't contain exactly one U+0078 LATIN SMALL LETTER X character, then this keyword doesn't represent anything. Abort these steps for that keyword.
Let width string be the string before
the "x".
Let height string be the string after the
"x".
If either width string or height string start with a U+0030 DIGIT ZERO (0) character or contain any characters other than characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then this keyword doesn't represent anything. Abort these steps for that keyword.
Apply the rules for parsing non-negative integers to width string to obtain width.
Apply the rules for parsing non-negative integers to height string to obtain height.
The keyword represents that the resource contains a bitmap icon with a width of width device pixels and a height of height device pixels.
The keywords specified on the sizes attribute must not represent
icon sizes that are not actually available in the linked
resource.
If the attribute is not specified, then the user agent must assume that the given icon is appropriate, but less appropriate than an icon of a known and appropriate size.
The following snippet shows the top part of an application with several icons.
<!DOCTYPE HTML> <html> <head> <title>lsForums — Inbox</title> <link rel=icon href=favicon.png sizes="16x16"> <link rel=icon href=windows.ico sizes="32x32 48x48"> <link rel=icon href=mac.icns sizes="128x128 512x512 8192x8192 32768x32768"> <link rel=icon href=iphone.png sizes="59x60"> <link rel=icon href=gnome.svg sizes="any"> <link rel=stylesheet href=lsforums.css> <script src=lsforums.js></script> <meta name=application-name content="lsForums"> </head> <body> ...
license"The license keyword may be used
with link, a, and area
elements. For link elements, it creates a hyperlink.
The license keyword indicates
that the referenced document provides the copyright license terms
under which the main content of the current document is
provided.
This specification does not specify how to distinguish between the main content of a document and content that is not deemed to be part of that main content. The distinction should be made clear to the user.
Consider a photo sharing site. A page on that site might describe and show a photograph, and the page might be marked up as follows:
<!DOCTYPE HTML> <html> <head> <title>Exampl Pictures: Kissat</title> <link rel="stylesheet href="/style/default"> </head> <body> <h1>Kissat</h1> <nav> <a href="../">Return to photo index</a> </nav> <figure> <img src="/pix/39627052_fd8dcd98b5.jpg"> <legend>Kissat</legend> </figure> <p>One of them has six toes!</p> <p><small><a rel="license" href="http://www.opensource.org/licenses/mit-license.php">MIT Licensed</a></small></p> <footer> <a href="/">Home</a> | <a href="../">Photo index</a> <p><small>© copyright 2009 Exampl Pictures. All Rights Reserved.</small></p> </footer> </body> </html>
In this case the license
applies to just the photo (the main content of the document), not
the whole document. In particular not the design of the page
itself, which is covered by the copyright given at the bottom of
the document. This could be made clearer in the styling
(e.g. making the license link prominently positioned near the
photograph, while having the page copyright in light small text at
the foot of the page.
Synonyms: For historical reasons, user agents
must also treat the keyword "copyright" like
the license keyword.
nofollow"The nofollow keyword may be
used with a and area elements.
The nofollow keyword indicates
that the link is not endorsed by the original author or publisher of
the page, or that the link to the referenced document was included
primarily because of a commercial relationship between people
affiliated with the two pages.
noreferrer"The noreferrer keyword may be
used with a and area elements.
It indicates that the no referrer information is to be leaked when following the link.
If a user agent follows a link defined by an a or
area element that has the noreferrer keyword, the user agent
must not include a Referer (sic) HTTP header
(or equivalent
for other protocols) in the request.
This keyword also causes the opener attribute to remain null if the
hyperlink creates a new browsing context.
pingback"The pingback keyword may be
used with link elements, for which it creates an external resource link.
For the semantics of the pingback keyword, see the Pingback 1.0
specification. [PINGBACK]
prefetch"The prefetch keyword may be
used with link elements, for which it creates an external resource link.
The prefetch keyword indicates
that preemptively fetching and caching the specified resource is
likely to be beneficial, as it is highly likely that the user will
require this resource.
There is no default type for resources given by the prefetch keyword.
search"The search keyword may be used
with link, a, and area
elements. For link elements, it creates a hyperlink.
The search keyword indicates that
the referenced document provides an interface specifically for
searching the document and its related resources.
OpenSearch description documents can be used with
link elements and the search link type to enable user agents to
autodiscover search interfaces. [OPENSEARCH]
stylesheet"The stylesheet keyword may be
used with link elements, for which it creates an external resource link that
contributes to the styling processing model.
The specified resource is a resource that describes how to present the document. Exactly how the resource is to be processed depends on the actual type of the resource.
If the alternate keyword is
also specified on the link element, then the link
is an alternative stylesheet; in this case, the title attribute must be specified on the
link element, with a non-empty value.
The default type for resources given by the stylesheet keyword is text/css.
Quirk: If the document has been set to
quirks mode and the Content-Type metadata of the external
resource is not a supported style sheet type, the user agent must
instead assume it to be text/css.
sidebar"The sidebar keyword may be used
with link, a, and area
elements. For link elements, it creates a hyperlink.
The sidebar keyword indicates
that the referenced document, if retrieved, is intended to be shown
in a secondary browsing context (if possible), instead
of in the current browsing context.
A hyperlink element with the sidebar keyword specified is a sidebar hyperlink.
tag"The tag keyword may be used
with link, a, and area
elements. For link elements, it creates a hyperlink.
The tag keyword indicates that the
tag that the referenced document represents applies to the
current document.
Since it indicates that the tag applies to the current document, it would be inappropriate to use this keyword in the markup of a tag cloud, which lists the popular tag across a set of pages.
Some documents form part of a hierarchical structure of documents.
A hierarchical structure of documents is one where each document can have various subdocuments. The document of which a document is a subdocument is said to be the document's parent. A document with no parent forms the top of the hierarchy.
A document may be part of multiple hierarchies.
index"The index keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The index keyword indicates that
the document is part of a hierarchical structure, and that the link
is leading to the document that is the top of the hierarchy. It
conveys more information when used with the up keyword (q.v.).
Synonyms: For historical reasons, user agents
must also treat the keywords "top", "contents", and "toc" like the
index keyword.
up"The up keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The up keyword indicates that the
document is part of a hierarchical structure, and that the link is
leading to the document that is the parent of the current
document.
The up keyword may be repeated within
a rel attribute to indicate
the hierarchical distance from the current document to the
referenced document. Each occurrence of the keyword represents one
further level. If the index keyword
is also present, then the number of up
keywords is the depth of the current page relative to the top of the
hierarchy. Only one link is created for the set of one or more up keywords and, if present, the index keyword.
If the page is part of multiple hierarchies, then they should be
described in different paragraphs. User agents
must scope any interpretation of the up
and index keywords together
indicating the depth of the hierarchy to the paragraph
in which the link finds itself, if any, or to the document
otherwise.
When two links have both the up and
index keywords specified together in
the same scope and contradict each other by having a different
number of up keywords, the link with the
greater number of up keywords must be
taken as giving the depth of the document.
This can be used to mark up a navigation style sometimes known as bread crumbs. In the following example, the current page can be reached via two paths.
<nav> <p> <a href="/" rel="index up up up">Main</a> > <a href="/products/" rel="up up">Products</a> > <a href="/products/dishwashers/" rel="up">Dishwashers</a> > <a>Second hand</a> </p> <p> <a href="/" rel="index up up">Main</a> > <a href="/second-hand/" rel="up">Second hand</a> > <a>Dishwashers</a> </p> </nav>
The relList DOM
attribute (e.g. on the a element) does not currently
represent multiple up keywords (the
interface hides duplicates).
Some documents form part of a sequence of documents.
A sequence of documents is one where each document can have a previous sibling and a next sibling. A document with no previous sibling is the start of its sequence, a document with no next sibling is the end of its sequence.
A document may be part of multiple sequences.
first"The first keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The first keyword indicates that
the document is part of a sequence, and that the link is leading to
the document that is the first logical document in the sequence.
Synonyms: For historical reasons, user agents
must also treat the keywords "begin" and
"start" like the first keyword.
last"The last keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The last keyword indicates that the
document is part of a sequence, and that the link is leading to the
document that is the last logical document in the sequence.
Synonyms: For historical reasons, user agents
must also treat the keyword "end" like the
last keyword.
next"The next keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The next keyword indicates that the
document is part of a sequence, and that the link is leading to the
document that is the next logical document in the sequence.
prev"The prev keyword may be used with
link, a, and area
elements. For link elements, it creates a hyperlink.
The prev keyword indicates that the
document is part of a sequence, and that the link is leading to the
document that is the previous logical document in the sequence.
Synonyms: For historical reasons, user agents
must also treat the keyword "previous" like
the prev keyword.
Other than the types defined above, only types defined as
extensions in the WHATWG Wiki
RelExtensions page may be used with the rel attribute on link, a,
and area elements. [WHATWGWIKI]
Anyone is free to edit the WHATWG Wiki RelExtensions page at any time to add a type. Extension types must be specified with the following information:
The actual value being defined. The value should not be confusingly similar to any other defined value (e.g. differing only in case).
linkOne of the following:
link elements.link element;
it creates a hyperlink
link.link element;
it creates a external
resource link.a and areaOne of the following:
a
and area elements.a and
area elements.A short description of what the keyword's meaning is.
A link to a more detailed description of the keyword's semantics and requirements. It could be another page on the Wiki, or a link to an external page.
A list of other keyword values that have exactly the same processing requirements. Authors must not use the values defined to be synonyms, they are only intended to allow user agents to support legacy content.
One of the following:
link" and "Effect on... a and
area" information should be set to "not
allowed".If a keyword is added with the "proposal" status and found to be redundant with existing values, it should be removed and listed as a synonym for the existing value. If a keyword is added with the "proposal" status and found to be harmful, then it should be changed to "rejected" status, and its "Effect on..." information should be changed accordingly.
Conformance checkers must use the information given on the WHATWG Wiki RelExtensions page to establish if a value not explicitly defined in this specification is allowed or not. Conformance checkers may cache this information (e.g. for performance reasons or to avoid the use of unreliable network connectivity).
When an author uses a new type not defined by either this specification or the Wiki page, conformance checkers should offer to add the value to the Wiki, with the details described above, with the "proposal" status.
This specification does not define how new values will get approved. It is expected that the Wiki will have a community that addresses this.