Cascading Style Sheets Level 1 Browser Incompatibilities: Teaches how to produce Web pages (formatted with Cascading Style Sheets Level 1) that display attractively on modern browsers and adequately on recent ones.
CSS Pointers Group: CSS Bugs and Workarounds: A list of CSS bugs in various browsers for various platforms. Some include demonstrations and screenshots, and workarounds.
CSS-Discuss: Wiki companion to the CssDiscussList mailing list. Includes tips and tricks and information on browser hacks, text sizing, and testing.
Navigator / Explorer Comparison: Comparison of how Netscape Navigator 4.x and Microsoft Internet Explorer for Windows render some examples of HTML and CSS.
QuirksMode - CSS contents and browser compatibility: Tests of CSS2 selectors, pseudoclasses and declarations, including compatibility information for Explorer, Mozilla, Safari, and Opera.
RichInStyle.com: CSS bug guide: List of CSS1 and CSS2 bugs in other Internet Explorer, Netscape, Opera versions, and a bug-by-bug guide on how to avoid them.
WestCiv - CSS1 and CSS2 Browser Support: Groups properties by level of support. Contains guide, tutorials, and articles.