The problem I am trying to solve is that of driving a validator. With a DTD you can, with a spec, I don't know. I have seen Microsofts attempt at defining a DTD for HTML5 -incomplete and inaccurate <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/
I can use the validator from validator.whatwg.org/.
Thanks for the sanswer