Draft Specification 2005-01-10
This specification is (C) 2004-2023 by the authors. However, the authors intend to submit (or already have submitted, see details in the spec) this specification to a standards body with a liberal copyright/licensing policy such as the GMPG, IETF, and/or W3C. Anyone wishing to contribute should read their copyright principles, policies and licenses (e.g. the GMPG Principles) and agree to them, including licensing of all contributions under all required licenses (e.g. CC-by 1.0 and later), before contributing.
Rel-Tag is one of several MicroFormats. By adding
rel="tag" to a hyperlink, a page indicates that the destination of that hyperlink is an author-designated "tag" (or keyword/subject) of the current page. Note that a tag may just refer to a major portion of the current page (i.e. a blog post). e.g. by placing this link on a page,
<a href="http://technorati.com/tag/tech" rel="tag">tech</a>
the author indicates that the page (or some portion of the page) has the tag "tech".
The linked page SHOULD exist, and it is the linked page, rather than the link text, that defines the tag. The last path component of the URL is the text of the tag, so
<a href="http://technorati.com/tag/tech" rel="tag">fish</a>
would indicate the tag "tech" rather than "fish".
rel="tag" is specifically designed for "tagging" content, typically web pages (or portions thereof, like blog posts).
rel="tag" is NOT designed for "tagging" arbitrary URLs or external content. There is demand for a general decentralized syntax for tagging URLs, and that is certainly something to think about, but this is not meant for that.
<dl class="profile"> <dt id="rel">rel</dt> <dd><p> <a rel="help" href="http://www.w3.org/TR/html401/struct/links.html#adef-rel"> HTML4 definition of the 'rel' attribute.</a> Here is an additional value.</p> <dl> <dt id="tag">tag</dt> <dd>Indicates that the referred resource serves as a "tag", or keyword/subject, for the referring page.</dd> </dl> </dd> </dl>
Tags are embedded in HTTP URIs in a well-defined manner so that the tag embedded in an HTTP URI can be mechanically extracted from that URI. Specifically, the last segment of the path portion of the URI contains the tag value. For example, the URI
contains the tag "foo".
Thus, for the purposes of comparing two HTTP URIs as tags, the last segment of the path portion should be extracted and only that value (that value of the tag) should be compared.
Need more formal language about comparison and extraction process.
The destination of a rel="tag" hyperlink is required to be a tag space (a place that collates or defines tags), where the last component of the URL is the tag, e.g.
is a URL for the tag "tech".
Since the only part of a tag space URL of which any structure is required is the last segment, a tag space URL can be hosted at any domain. Authors may choose to link to a tag at a particular tag space in order to provide a specific meaning. E.g. a tag for technology could link to:
Spaces can be encoded either as
%20. Unicode characters are encoded as specified in RFC 3986. For example:
<a href="http://technorati.com/tag/Sant%C3%A9+et+bien-%C3%AAtre" rel="tag">Santé et bien-être</a>
Tags Are Visible Metadata
rel="tag" hyperlinks are intended to be visible links on pages and posts. This is in stark contrast to meta keywords (which were invisible and typically never revealed to readers), and thus is at least somewhat more resilient to the problems which plagued meta keywords.
Making tag hyperlinks visible has the additional benefit of making it more obvious to readers if a page is abusing tag links, and thus providing more peer pressure for better behavior. It also makes it more obvious to authors, who may not always be aware what invisible metadata is being generated on their behalf.
As a result the invisible tag link syntax variant:
<link rel="tag" href="..." /> SHOULD NOT be supported by implementations.
This section is informative.
The following implementations have been developed which either generate or parse rel-tag links. If you have a rel-tag implementation, feel free to add it to the top of this list. Once the list grows too big, we'll make a separate wiki page.