XHTML Tutorial - 8 pages, including introduction, differences from HTML, syntax, DTD, examples, validation, and a quiz. Also, reference pages provide lists of tags and attributes.
Computer Torture - Lists of brief guidelines for XHTML and for XHTML-Basic. [Author and qualifications are not identified.].
Joe Burns: XHTML - Short page introducing the relationships between XHTML and HTML..
Simon Jessey: Learning XHTML - 5 parts including: page structure; lists; hyperlinks and text formatting; tables; and forms. Suitable for beginners, without prior knowledge of HTML..