This post is really for novice users who would like to learn how to create web pages with (X)HTML, instead of using those horrid WYSIWYG editors. I've recently updated an XHTML tutorial I wrote a few years ago. It offers a very gentle introduction to XHTML in five basic lessons.
Over the years, it has been quite popular (4th hit on Google), but it hadn't been revised since the release of the Second Edition of XHTML. All you need is a text editor and a browser.