Depending on which statistics you look at, less than 10% of Web surfers still use 4.x or less browsers. That leaves versions 5.x and 6.x of Explorer on PC (and 5.x versions on Mac) with the lion's share. Other new browsers like Netscape/Mozilla, Opera and iCab still have relatively little support. In percentage terms, about 3% in total.
Of course, these statistics are generalised, it's your own site stats that really count.
If we now look at how the majority of browsers handle CSS, we are talking about how consistently MS Explorer 5.x (Windows and Mac) and 6.x (Window only) do it. Well, yes, there is good style sheet support in all of them but consistent - mmm.
IE 6.x is the nearest one of these to being standards compliant - but compared to the other so-called 'compliant' browsers, still has some differences that can trap the unwary. You also have to ask yourself, if the vast majority are using IE and only a small minority are using standards - what are you supposed to do?
Rather than re-iterate all the differences in CSS1 and CSS2 support in various browsers, I will just point you to this excellent resource by the folks at WestCiv .
John Allsopp and Maxine Sherrin have probably done more for the furtherance of style sheets than anybody else on the planet. Their Style Master style sheet editor is by far the easiest to use and most indispensable tool there is for any Web designer. It doesn't pretend to do everything that programs like Dreamweaver, GoLive or FrontPage do but what it does do, it does without peer.