In the early days of the Web, O'Reilly's HTML: The Definitive Guide was the reference for HTML. It's still the best available, but now in its fourth edition, HTML & XHTML is beginning to show its age. While I would still recommend it for Web professionals who need a reference, I'm not sure I would suggest it to folks just starting out.

The art and science of Web creation are going through fundamental changes. HTML is slowly being supplanted by XML, and structure and presentation are finally and irrevocably being separated. I would suggest that within the next three years this book will need to be replaced with a similar broad introduction to Web authoring, something that covers the basics of XML, CSS, XSL and JavaScript, with little mention at all of HTML. But we will see. In the meantime, this is still the book to grab when you have an HTML problem to solve. The appendices, in particular, are invaluable.