There are a number of standards for supporting 'tags' in RSS including:

dc:subject RSS 2.0 'category' mod_taxonomy

How are we going to build a generic API that supports all systems currently in use?

RSS 2.0

http://blogs.law.harvard.edu/tech/rss#ltcategorygtSubelementOfLtitemgt

<category> sub-element of <item> 

<category> is an optional sub-element of <item>.

It has one optional attribute, domain, a string that identifies a categorization taxonomy.

The value of the element is a forward-slash-separated string that identifies a hierarchic location in the indicated taxonomy. Processors may establish conventions for the interpretation of categories. Two examples are provided below:

<category>Grateful Dead</category>

<category domain="http://www.fool.com/cusips">MSFT</category>

You may include as many category elements as you need to, for different domains, and to have an item cross-referenced in different parts of the same domain.

Delicious

<taxo:topics>
<rdf:Bag>
<rdf:li resource="http://del.icio.us/tag/programming"/>
</rdf:Bag>
</taxo:topics>

Technorati

Technorati tags seems proprietary:

To contribute to this tag, just make a post to your blog about met and include a link to this page like so:

<a href="http://technorati.com/tag/met" rel="tag">met</a>

Furl

Furl also supports tags:

http://www.furl.net/members/rss.xml?topic=technology

And the resulting XML would include:

<category>Technology</category>
<dc:creator>ggussis</dc:creator>
<guid isPermaLink="true">http://www.furl.net/item.jsp?id=1741406</guid>
<pubDate>Wed, 19 Jan 2005 19:21:32 GMT</pubDate>
<furl:rating>3</furl:rating>
<furl:clipping/>

Ideal HTML specification

http://www.peerfear.org/rss/permalink/2005/01/18/TechnoratiTagAPIIsBroken/

Interface

/**
  * @param tagname The local name of the tag.  IE 'linux'
  * @param URI the URI ot the tag (if invalid null).  This is used for tagging 
  * systems that use URI.  When these systems use URIs the tagname is the last
  * path component of the URI.
  */  
 onTag( String tagname, String uri ): void

TagMetaDataEvents (last edited 2009-09-20 23:48:44 by localhost)