3.6. Naming Rules, XHTML Modularization 1.1, W3C Working Draft 5 July 2006

I've just come across this section of XHTML Modularization 1.1:

  Only XHTML Host Language conforming documents should begin the
  public text description with the token XHTML.

Now, that isn't a "SHOULD", but it seems to be coming rather close to
redefining http://www.w3.org/TR/xml/#dt-doctype and I can't find any
mention of the XHTML Modularization spec there.

Is it appropriate for authors of XML documents to have the rules for
authoring XML documents modified by an unreferenced specification?

-- 
David Dorward                                      http://dorward.me.uk