SitePoint recently sent me a link to a site called CSS Reference. As they describe it:
In this free online reference, the entire CSS language is clearly
and concisely explained, including browser compatibility, working
examples, and easy-to-read descriptions.
The reference has been written by two of the world's most renowned
CSS experts -- Tommy Olsson and Paul O'Brien -- so you know it's
accurate, up to date, and best practice.