I thought the Feed Validator’s ‘foo’ media type is not specific enough warning page was particularly useful/interesting.

RSS feeds should be served as application/rss+xml. Atom feeds should use application/atom+xml. If this is not possible, make sure that one of the following set of markers is present in the first 512 bytes of your feed

• <rss
• <feed
• <rdf:RDF AND http://www.w3.org/1999/02/22-rdf-syntax-ns# AND http://purl.org/rss/1.0/

This post first appeared on From the Belly of the Beasts, a weblog from some of the people who build O’Reilly websites.