http://www.digital-web.com/articles/html5_xhtml2_and_the_future_of_the_web/ Quotes: The fact that Internet Explorer doesn’t really support XHTML as XML in any way, and the problems XML can cause when not all tools in the authoring chain are XML tools, means that there has been little incentive for using XML on the web. This is compounded by search engines not indexing XHTML as XML documents; very few XHTML authoring tools for XML; very few CMS or blogging tools supporting XML correctly all the way from input through database to generation; and very few ad suppliers supporting XML. While HTML 4.01 is formally SGML-based, HTML5 accepts the reality of all browsers using error-correcting tag-soup parsers, and instead describes a specific non-SGML parsing model that includes a defined error correction model. Turning an HTML 4.01 document into HTML5, on the other hand, is in most cases just a question of replacing the DOCTYPE declaration. If a document doesn’t use any of the new elements or APIs introduced by HTML5, the browser just sends it to its tag-soup parser. For most current content-management systems and authoring tools, the change to generate HTML5 instead of HTML 4.01 is simple, and the new HTML5 features can be added to them easily. In addition, many of the new HTML5 features can be emulated using JavaScript for browsers that don’t support them, allowing for a gradual change from HTML 4.01 to HTML5.