I often hunt for official specifications for which CSS property is supported on which version of a browser. The fastest and best source for that is Sitepoint CSS Reference. But sometimes I like to verify it with the official documentation. So that I can have more time to tweet be more productive, here are the links to the official documentation of CSS support for all mainstream browsers.

• Opera (Look for Opera Presto 2.3 for CSS features included in Opera 10.1)
• Safari (lists what is available in which version of Safari, per property. Be warned, some of those listed as “supported” are really not).
• IE (separate list of supported CSS properties per browser version).
• Firefox (lists what is available in which version of Firefox, per property)

If you are looking to use an upcoming standard (like CSS3, HTML5) you can also refer to this excellent list of cross-browser support for each feature or the respected Quirksmode Compatibility Tables.