XHTML 1.0 is the latest reccomendation from the World Wide Web Consortium. It replaces their previous reccomendation, HTML 4.01.
XHTML is all very well as a stand-alone standard but can you use it? I say yes, and here’s a list of all XHTML tags and which ones you can safely use. For how XHTML renders on your browser see the all valid XHTML tags document.
Further reading: W3C Markup Language Home Page