CSS2 became a recommendation in May of 1998. It extends CSS1, so CSS1 is a subset of CSS2 with some very minor changes. Support for CSS2 was almost nonexistent in Netscape Navigator 4.x, and very limited in Internet Explorer 4.5, 5 and 5.5 for Windows.
Internet Explorer 5 for Macintosh and 5 and 6 for Windows has quite a bit of support for CSS2, though falls quite a way short of full support.
Netscape 6, and Opera 5 both have quite substantial support for CSS2.