L3 Core, Section 1.3.2, xmlns binding

 From http://www.w3.org/TR/DOM-Level-3-Core/core.html#domURIs


    Note: In the DOM, all namespace declaration attributes are by
    definition bound to the namespace URI:
    "http://www.w3.org/2000/xmlns/". These are the attributes whose
    namespace prefix orqualified name is "xmlns". Although, at the time
    of writing, this is not part of the XML Namespaces specification
    [XML Namespaces], it is planned to be incorporated in a future revision.

It appears that Namespaces 1.1 introduces that namespace URI and could 
be explicitly mentioned.