Web Design in a Nutshell, 3rd Edition

Other Link Relationships

XFN was the first such popular extension of the rel attribute, and others followed soon after. The Creative Commons is a non-profit organization that encourages authors and artists to share their digital works using a standard set of online licenses. Authors can indicate that a document is published under a Creative Commons license by linking from the document to the license and adding a rel attribute with value of license:

<arel="license" href="http://creativecommons.org/licenses/by/2.5/">...</a>

Search engines, including both Yahoo! and Google, recognize such license links and offer the ability to search for content available under such licenses.

The practice of visibly “tagging " content on the Web—in particular, links and photos—inspired the creation of the tag relationship value to indicate that the destination of a link represents a “tag” for the current document or portion thereof. A blog post can be tagged as being about “CSS” by including the following visible tag link inside the contents of the post:

<arel="tag" href="http://technorati.com/tag/css">CSS</a>

Newer search engines, such as Technorati and Ice Rocket , recognize such tag links and have incorporated tagged content into their search results and other services.

To help combat web spam, publishers and search engine companies developed the nofollow extension. Many automatic and third-party generated hyperlinks are published with rel="nofollow", which search engines use to afford less weight to ...