Brian Del Vecchio wrote a long entry about about splicing a RSS feed by tag and how aggregator developers can create more tag-aware applications. It is possible within the RSS specification, but most feed authors have overlooked how to properly declare tags within their feeds. Flickr does not include categories with its RSS 2.0 feeds and Del.icio.us feeds use Dublin Core subject to provide space-separated tags.

Before the feed aggregators can make your life easier we need to all do a better job as content producers to provide valuable information for our users.

RSS 2.0

Each RSS 2.0 feed has an optional category sub-element with an optional domain attribute to designate a specific categorization taxonomy. You can have as many category elements as you need to with mixed or no domains.

1. <category domain=”http://del.icio.us/tag”>foo</category>
2. <category domain=”http://www.flickr.com/photos/tags”>foo</category>
3. <category domain=”http://www.technorati.com/tag”>foo</category>

Using the category element with the domain attribute set to the best definition of the tag’s context allows for easy recognition of tags within the aggregator. It also allows a user to search their own database or create smart lists for all items with Flickr or Technorati tags, or build into the application a list of known tagging domains for special status.

IETF Atom

The current Atom draft provides similar capabilities through the use of category elements with a scheme URI.

1. <category scheme=”http://del.icio.us/tag” term=”foo” label=”Del.icio.us tag for foo”/>
2. <category scheme=”http://www.flickr.com/photos/tags” term=”foo” label=”Flickr tag for foo”/>
3. <category scheme=”http://www.technorati.com/tag” term=”foo” label=”Technorati tag for foo”/>