CSS-Tricks has got to be, hands down, one of the most interesting and useful resources on the web I have ever found. As CSS has evolved, so too have the methods of doing things, and CSS-Tricks keeps you up to date with the latest.
The site also has an ‘almanac’ feature, where you can lookup CSS selectors and properties. They then show you how to properly use the selectors and properties to achieve the results you’re looking for.
There are also forums available where you can discuss different methods, ask for help, or just to discuss all things HTML+CSS.