Validator complains because it cannot determine validation mode from document type

I am developing some new content for the XHTML 2 working group at 
http://www.w3.org/MarkUp/Drafts/Overview.html

When attempting to validate this, the validator complains that the 
referenced DOCTYPE, admittedly a private, contrived markup language that 
the validator does not know about explicitly, is being served as 
text/html and it cannot decide how to parse it.

Now... I know that I could change the rules so that a requestor who 
accepts application/xhtml+xml would get the document with that media 
type, but...  I think of the document starts off with an "xml" 
declaration, e.g., <?xml ... ?>, then the validator should assume that 
the document is to be validated in XML mode.  

Am I missing something here?