In response to the question: I agree with cwilbur that there’s little practical difference between XHTML and HTML, but I would say that XHTML it’s somewhate more strict than HTML, at least in one aspect: the elements should be correctly nested, which implies documents have to be well formed as in XML. There are other differences between HTML 4 and XHTML 1 which you can see on that link.
On the other hand, XHTML as XML is a big issue, it would actually deserve an entire article on the subject (which, by the way, I wrote on my blog [in spanish, but you could check the links that I used as source to get an idea]), but in two lines, there are some things that should be clarified:
· XHTML 1.0 may be served as text/html, but should be served as application/xhtml+xml (source). If you serve it as application/xhtml+xml, your document has to be well formed, otherwise the browser will just display an error message.
· If you serve XHTML as text/html, you wouldn’t get the benefits of XML parsing, such as faster rendering or integration of other XML languages (for instance, MathML), but instead it will be parsed as HTML… and I think that in most browsers that just means “making some sense out of tag soup”.
All in all, XHTML just doesn’t make much sense given the very strict XML parser, and it’s practically senseless serving it as text/html
HTML 5 it’s not my personal favorite (I think it lacks the clear focus of XHTML and they’re deviating from the minimization of elements/attributes which IMHO would constitute a better markup language), but at least it’s realistic in the sense that there are millions of web sites that just don’t have good code, and that there should also be some way for browsers to display a document even when the code isn’t perfect. In other words: backwards compatibility.
And, by the way, you can still use CSS with HTML, in fact, there’s absolutely no reason not to.
Of course, even when this is a somewhat lengthy response, it not even near to be “definitive”, since I haven’t even considered other factors such as Quirks/Standards mode, and many others.