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.

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.

Navigator / Explorer Comparison
Comparison of how Netscape Navigator 4.x and Microsoft Internet Explorer for Windows render some examples of HTML and CSS.

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.

WebReview CSS2 Selectors Chart
Shows CSS2 Selectors support on Win95 (in Nav4.6, IE5, and Opera 36) and Mac (in Nav4.5, IE4.5). Registration required.

WebReview - Style Sheet Reference Guide
Offers browser compatibility charts, links to articles, and an FAQ. Registration required.

Bosh's CSS Info
CSS filters (hacks) and browser bugs.

CSS2 tests
Tests of new CSS2 selectors, pseudoclasses and declarations, including compatibility information for Netscape 4+, Explorer 4+ (Win and Mac), Opera 5 and Konqueror.

Grading the Browsers for Style Sheets
This article is an thorough and informative exploration into how CSS works in different browsers.