Rating: 

I think it is the most complete reference book on (X)HTML ever, explaining every tag and extension this language has. The only (little) disappointment I had with it is that it doesn't teach good and strong standards behaviour in some points, but apart from that, it's an essential reference book.

If you use it as a complement to Head First HTML with CSS and XHTML (http://www.oreilly.com/catalog/hfhtmlcss/) , you'll get the whole and complete standards-based XHTML experience.