html, dhtml, xml, xhtml definition
For readers unacquainted with either term, HTML is the set of codes (that's the markup language) that a writer puts into a document to make it displayable on the World Wide Web. HTML 4 is the current version of it. XML is a structured set of rules for how one might define any kind of data to be shared on the Web.
It will encourage web authors to use more XML features and make their JavaScript interoperable with real XHTML applications. (Note that XHTML documents which behave correctly in both application/xhtml+xml and text/html environments are sometimes known as 'polyglot' documents.)