HTML 5 differences from HTML 4

http://www.w3.org/TR/html5-diff/

A vocabulary and associated APIs for HTML and XHTML - the big spec
http://www.w3.org/html/wg/html5/

Planet HTML
http://people.w3.org/mike/planet/html5/

HTML 4.01 test suite
http://www.w3.org/MarkUp/Test/HTML401/current/tests/

HTML 4.01 specification including index of elements, index of attributes, and general index
http://www.w3.org/TR/REC-html40/cover.html#minitoc