XHTML

The evolution of HTML has essentially stopped. Instead, HTML is being replaced by a new language, called XHTML. XHTML is in many ways similar to HTML, but is designed to work with the new eXtensible Markup Language, or XML, that will soon serve as the core language for designing all sorts of new Web applications, in which XHTML will be only one of many "languages." But, XHTML is designed to work with these other languages, so that different documents, in different languages, can be easily mixed together.

XHTML is significantly better than HTML, as it encourages good practice. Where it doesn't allow things that HTML does, it's likely there's a good reason.. for instance not using target="_blank". These things then in turn impact on the sites accessibility, consistency, search engine performance etc etc.