Excerpt from the recently released W3C XHTML/HTML FAQ:

CSS has a lot of special rules that only apply to HTML. Do these also apply to XHTML?
No. CSS rules that apply only to HTML, apply only to documents that are delivered as text/html.

First, I warned Steven Pemberton the 31-aug-2003 about using the word "rule" here. A "CSS rule" is something too meaningful for us to reuse it like he did. But he did not make the change.

Second, if I read this FAQ entry well, and if I compare it to the CSS 2 specification, it implies that the class selector does not apply to XHTML if it is not served as text/html!!! The div.warning applies to <div class="warning"> only in HTML documents according to the spec.

Sorry to say, but this is totally ridiculous, an author having almost never any form of control on the future mimetype of the instance he is authoring. I am ready to bet that browsers will not comply with this decision and will apply the class selector to xhtml instances whatever is the mimetype.