I started learning CSS at this site - http://www.subcide.com/articles/creating-a-css-layout-from-scratch/ and often go back to read his articles. He posted an article http://www.subcide.com/articles/pure-css-twitter-fail-whale/ about Twitter going down. If you have a WebKit-based browser the project he developed works great. It shows off what can be done with CSS. Check it out.