XHTML is intended for web pages, and similar narrative documents meant for humans to read. Because it's XML based it's also easier for software to read. However it's not a general purpose solution for data formats like XML is. XHTML is discussed in Item 23, "Reuse XHTML for generic narrative content", and Item 46, "Tidy Up Your HTML"
Elliotte Rusty Harold<br />Author of <a href="http://cafe.elharo.com/web/refactoring-html/" target="_blank" rel="nofollow">Refactoring HTML</a>