If you haven’t already seen it, the good folks at SitePoint have put together a CSS Reference and made it publicly accessible here.
“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.”
Its other nice feature is the ability to accept user contributed notes.