validar html5 en xhtml

[sgardes]

Gracias por el aporte Bonez.
Pero no ha funcionado
W3C no lo toma como valido, aqui su respuesta al intentar validar la pagina index.html con tu codigo en primera linea.

Fatal Error: cannot find "xhtml1-strict-with-html5-microdata.dtd"; tried

I could not parse this document, because it makes reference to a system-specific file instead of using a well-known public identifier to specify the type of markup being used.

You should place a DOCTYPE declaration as the very first thing in your HTML document. For example, for a typical XHTML 1.0 document: