This helpful introduction to HTML is from the UW's own Student Access and Computing Group.
The Web Design Group was founded to promote the creation of non-browser specific, non-resolution specific, creative and informative sites that are accessible to all users worldwide. To this end, the WDG offers material on a wide range of HTML related topics. They hope that with this site as a reference, you will be able to create Web sites that can be used by every person on the Internet, regardless of browser, platform, or settings.
This hypertext article explains why XHTML was created, how it is different from HTML, and what you can do with it.
This tutorial offers a thorough explanation of XHTML.
A nice reference for XHTML tags.
This is the offical recommendation for XHTML 1.0 from the World Wide Web Consortium.