The \"old\" HTML/XHTML standards have a DTD (Document Type Definition) defined for them:
HTML 4.01 http://www.w3.org/TR/html401/sgml/dtd.html XHTML 1.0 http://ww
I have created an HTML5 DTD for use in my PHP XML projects. It ain't beautiful, but it works with well-formed XHTML5 (that is, HTML5 expressed as XML).
You can grab it from my bitbucket account here:
https://bitbucket.org/kashbridge/dtd/overview
Enjoy!