You'll probably find that harmless-looking XHTML DTD is declaring a
default namespace for you - a practice which should be punishable by
flogging.

<!-- the namespace URI designates the document profile -->

<!ELEMENT html (head, body)>
<!ATTLIST html
  %i18n;
  xmlns       %URI;          #FIXED 'http://www.w3.org/1999/xhtml'
  >

However, editing the xmlns attribute out of the DTD doesn't fix the
problem.

Removing the DocType Declaration from the instance fixes it. I thought
there might be a further entity reference with another copy of the
namespace declaration but running spam to expand the whole caboodle
and grepping for xmlns says not.