HTML 5 incorporates HTML and XHTML. In order to follow the standards correctly, the browser will parse as HTML if text content is received or as XHTML if XML content received. This is governed by the content type broadcasted by the web server.
In reality, IE would freak out if you used the XML content type, and as all browsers will still parse XHTML 5 syntax even if the content type is text. I use XHTML syntax with HTML5, delivered as text.