MSDN, O'Reilly markup quality
Thread on Greasemonkey
Mark on markup quality
http://www.w3schools.com/tags/tag_a.asp
http://www.w3schools.com/tags/tag_address.asp
Open-ended links
Microformats: http://developers.technorati.com/wiki/MicroFormats
Using existing XHTML markup for more semantics: http://tantek.com/presentations/2005/03/elementsofxhtml/