The XHTML DTD: a Hybrid and XML and HTML

XML is a little different from traditional HTML in that is much more strict. In HTML, you can leave certain tags out and use a document structure that is inconsistent, for example, you can use an <li> tag without a closing </li> element, which can make programmatically reading (parsing) the document code a painful process for anyone writing custom programs.

XHTML, however, follows all the rigid syntax rules of XML but it uses no custom elements: all elements and attributes come from HTML. So any application, like a Web browser, that can read HTML can read XHTML. And any application design to read XML can also read XHTML. You get the best of both worlds. And it's all possible because of the XHTML DTD, which is really an XML DTD that defines all its elements based on the HTML DTD.