When learning CSS I relied heavily on Index Dot Css, a comprehensive guide to CSS support in browsers. It’s still very useful, although sadly not updated since October 2003. After that I turned to W3Schools CSS Reference (when I didn’t have my copy of the CSS Pocket Reference to hand); it’s nicely laid out and fairly comprehensive, although with some curious omissions (no list of selectors?) and no way of telling if it’s ever updated.
Now there’s a new face on the scene; Sitepoint’s CSS Reference is out of Beta, and it looks good; nicely laid out, up-to-date browser support lists, and allows feedback from users. I haven’t used it extensively yet, but it looks pretty impressive at first glance.