After you're familiar with CSS and HTML, I would suggest you move into XHTML. I find the semantics to be cleaner. HTML and XHTML are pretty much the same except they enforce HTML rules more strictly.
XHTML cannot be served as XHTML because the most-used browser wouldn't know what to do with it. IE will offer to download the file is it is served as XHTML.
If it is not served as XHTML, then all browsers will treat it as badly formed HTML, possible going into quirks mode as a result.
There is currently no point in using XHTML.