This is an excellent introduction to HTML 4.01. However, the XHTML 1.0 elements are less well covered and XHTML 1.1 is mentioned only in a few paragraphs. Although, to be fair, the differences from XHTML 1.0 are slight and only of interest to a few.
However, my main complaint is that the index is almost of no use for locating actual tags and their usage in the text. Additionally, the ordering of the topics means that this book should be seen more as a series of tutorials than as a reference from which information can be easily extracted.
This really is the definitive guide. The younger one is, probably the easier it is to comprehend. For an old timer like me, it does take a little longer !! However, the only publication which is worth having.