A List Apart
http://www.alistapart.org/ 
Cornerstone of web design.

HTML Utopia - Designing Without Tables
http://www.sitepoint.com/article.php/379
The foundation of the book by the same title

Style Sheet Reference Guide
http://www.webreview.com/style/
Most comporehenseive table of CSS compatability.

The Anybrowser Campaign Site Design Guide
http://www.anybrowser.org/campaign/abdesign3.html
A site dedicated to making sure your HTML displays in all browsers.

Glish.com CSS Layout Techniques
http://glish.com/css/
Some good resource links, seems to bey dying, many broken links.

The Layout Reservoir - BlueRobot
http://bluerobot.com/web/layouts/default.asp
Code repository for CSS layouts.

Little Boxes at the Noodle Incident
http://www.thenoodleincident.com/tutorials/box_lesson/boxes.html

CSS, Cascading style sheets, HTML, website tips at Websitetips.com
http://www.websitetips.com/css/index.shtml