rel-tag

Draft Specification 2005-01-10

• If you’re looking for rel="me", see: rel-me.

Editors/Authors

Tantek Çelik

Kevin Marks

Concept

Derek Powazek

Incorporated into

W3C HTML5

Copyright

This specification is (C) 2004-2024 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.

• Tantek: I release all my contributions to this specification into the public domain and I encourage the other authors to do so as well.
• Kevin Marks: I release all my contributions to this specification into the public domain and I encourage the other authors to do so as well.
  • When all authors/editors have done so, we can remove the MicroFormatCopyrightStatement template reference and replace it with the MicroFormatPublicDomainContributionStatement.

Patents

This specification is subject to a royalty free patent policy, e.g. per the W3C Patent Policy, and IETF RFC3667 & RFC3668.

Abstract

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) for 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".

Scope

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 external to the current page, but this is not meant for that. See xFolk and hReview for ways to tag arbitrary URLs.

If you need to define tags as part of a more specialized format, rel="tag" is the recommended way to do so, and xFolk, hReview, hCard, hCalendar and hRecipe all do this.

XMDP profile

See rel-tag-profile.

Tag Spaces

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 (after the final "/" character) contains the tag value. For example, the URI

http://www.example.com/tags/foo

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 segment of the path of the URL is the tag, e.g.

http://technorati.com/tag/tech

is a URL for the tag "tech".

Tags may only be placed in the URL path, and only in the last segment of the path. Tags may not be placed in query parameters or fragment identifiers. e.g.

http://technorati.com/tag/tech?tag=fish#emu

is still a URL for the tag "tech", not "fish" or "emu".

Since the only part of a tag space URL of which any structure is required is the last path 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:

  http://en.wikipedia.org/wiki/Technology

Trailing slashes in tag URLs are ignored, that is:

  http://technorati.com/tag/Technology/

as a rel-tag URL is treated as:

  http://technorati.com/tag/Technology

Encoding issues

Spaces can be encoded either as + or %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>

Note that if using Wikipedia as a tagspace, as discussed above, you should use %20 as they remap '+' to %2B, causing a page with a plus sign in the title (which usually does not exist) to appear.

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.

Examples in the wild

This section is informative. The number of rel-tag examples in the wild has expanded far beyond the capacity of being kept inline in this specification. They have been moved to a separate page.

See rel-tag Examples in the wild.

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, please add it to the top of this list. Once the list grows too big, we'll make a separate wiki page like rel-tag-implementations.

• Degoli is a simple experimental semantic web seach engine which is crawling rel-tag data. It orders search results by rel-tag data and displays a navigation tree containing relationships among tags.
• b2evolution A blog platform which generates rel-tag for all tags on blog posts (starting from v3.x).
• Textcube.org makes a blogging tool, Textcube which generates rel-tag for tags on blog posts.
• HashTags tags Twitter posts, converting "#example" into a tag of "example"
• ikiwiki generates rel-tag for all tagged pages (as of version 2.6)
• HubTag helps users find a unique tag across the web for their event - eg. JohnAndBettysWeddingWimbledonMarch07
• Necctar is a search engine exclusively based on tag microformat parsing and processing. Necctar uses tags entered by bloggers to index the world wide web
• Nutch has a rel-tag parser committed to their svn repository.
• Dreamweaver Extension suite from the Web Standards Project enables rel-tagging from within Dreamweaver 8.
• Scooch slide show creator allows authors to generate rel-tags and group slide shows by rel-tag via a list or cloud with tag usage count.
• The Freetag plugin for the Serendipity blog software supports rel-tags on a per-entry basis, as well as inside of its tag clouds. (The Freetag plugin is available inside of SPARTACUS)
• pnh_mf is a plugin for Textpattern that supports embedding rel-tags and other microformats in templates and blog posts. Written by Chris Casciano.
• tru_tags is a plugin for Textpattern that supports rel-tagging blog posts via the Keywords field.
• ClothesOnline uses rel-tag for categorizing shops and brands, for example: Canada Goose.
• LiveJournal - see also their FAQ regarding their tags support
• TagsLinks Turn each tag into links that let you find related content on tagging services.
• OctoFinder uses rel-tag for all live news tag clouds.
• Tag plugin for WordPress
  • Note that some sites using WordPress (http://microformatique.com/ for instance) are getting incorrect tags. The tag is ?cat=12 instead of the actual tag value.
• Tag plugin for Blosxom
• Technorati first implemented rel-tag in its Technorati Tags service. Technorati indexes rel-tag tags.
• Greasemonkey script for Firefox that generates tags for Blogger
• rel-lint is a validation tool by Drew McLellan that will validate existence of rel-tag attributes.

articles

This section is informative.

Articles about rel-tag, most recent first. When this section gets too big, we can move it to rel-tag-articles.

• (undated) SitePoint reference: rel- Microformats / tag by Ian Lloyd

References

Normative References

• HTML 4
• XHTML 1
• XMDP
• RFC 3986 specifies URL syntax. Section 3.3 specifies URL paths and path segments.

Informative References

• hReview uses rel-tag for tags and scalar tags
• xFolk uses rel-tag to build a distributed remote resource tagging construct
• Attention.XML uses rel-tag for reader tagging of pages, posts, feeds
• hCard can use rel-tag for categories
• hCalendar can use rel-tag for categories
• Using Technorati Tags
• Contributed from http://developers.technorati.com/wiki/RelTag
• Know your rel-tag at microformatique.com

Discussions

• Feedback is encouraged on the rel-tag-feedback page.
• See also blogs discussing this page.
• History: How Tags Happened at Technorati by Derek Powazek

Q&A

• If you have any questions about rel-tag, check the rel FAQ first for general rel attribute questions, then check the rel-tag FAQ, and then if you don't find answers, ask your question on the microformats-discuss mailing list.

Related pages

• rel-tag
• rel-tag-faq
  • See also rel-faq
• rel-tag advocacy - encourage others to use rel-tag.
• rel-tag-spaces - sites suitable as using as targets for rel-tag links.

The rel-tag specification is a work in progress. As additional aspects are discussed, understood, and written, they will be added. These thoughts, issues, and questions are kept in separate pages.

• rel-tag-feedback - general feedback (as opposed to specific issues).
• rel-tag-issues - specific issues with the specification.
• rel-tag-other-evidence - tagging schemas other than those used in rel-tag