What would an HTML5 theme offer that an XHTML1 theme couldn’t? Semantics and other great new elements, which make html code better and more suitable for CMS.
This article is a great example, which elements you can use in a HTML 5 theme for WordPress
https://www.ibm.com/developerworks/library/x-html5/
HTML 5 is the future of the web and in the near future most of the web browsers will support HTML 5. At the present Firefox 3.5 and browsers based on Webkit core support some HTML 5 elements.
The main idea is to do this for my diploma work and also to improve WordPress.