I agree with Elliotte. I think the key point of his post is "I certainly
don't want to rely on CSS for anything important." I would support the idea
of going one direction with the XHTML stylesheets and another direction with
the HTML stylesheets. That way, those for whom backwards compatibility is
not an issue can push the envelope all the way with XHTML and CSS2, no holds
barred, and the rest of us can continue as before.
Yes, browsers that support XHTML generally support CSS as well.
However, this tends to only be >= IE6/Mozilla 1.0/Netscape 7