HTML CSS

HTML5

HTML5 Rocks HTML5 Bible Semantic HTML (HTML5)

Browser Compliance: Up to date info on browser manufacturers compliance with HTML5 & CSS3 standards, and a couple others.

HTML - General

QuirksMode - Browser Compatibility HTML URL Encoding Reference

CSS - General

CSS Reset: IMHO, good to include if you want to 'reset' the default styles of whatever browser a user happens to be using when they see your site.