CSS Reference
W3C Cascading Style Sheets Home Page
The W3C’s CSS home page provides links to tutorials, specifications, test suites, and other resources:
Mezzoblue CSS Cribsheet
Dave Shea provides this helpful CSS cribsheet in an attempt to make the design process easier, and provides a quick reference to check when you run into trouble:
Position Is Everything
This site includes a catalog of CSS browser bugs along with explanations of how to overcome them: