CSS2 Browser support matrix
Mac Edition offers a Abridged Guide to CSS2 Support that documents in a matrix which browsers support what. Keep in mind that Gecko is Mozilla, which is Netscape 6 and up as well as Firebird (an insanely fast and standards compliant browser). It is also good to note that Gecko-based browsers render the same regardless of operating system, unlike Internet Explorer, which is different depending on operating systems. The matrix also includes how the browsers hold up to various hack tests, most of the test are for box model layout (Tantek and Owen Briggs tests included).