The new iCab beta (registered users only) supports “CSS 2.0/2.1 probably more complete than most other browsers,” and it works on systems all the way back to Mac OS 8.5 (the first version to include ATSUI). The CSS support does seem to be much improved, but it was not evident to me how it’s better than in Internet Explorer 5 or Safari, and there are some areas where it’s clearly worse.
Update: I have now seen some pages that iCab renders better than IE 5. One area where iCab was noticeably worse, the failure to honor
text-decoration: none, can be adjusted in the preferences.