Draft Specification 2005-01-10
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 has the tag "tech".
The linked page SHOULD exist, and it is the linked page, rather than the link text that defines the tag.
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>
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:
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.