Language of a node and HTML+RDFa JavaScript implementations

Section 3.3 of HTML+RDFa says:
 
    RDFa processors MUST use the mechanism described in The lang and xml:lang 
    attributes section of the [HTML5] specification to determine the language of a node.
    
But a node's language depends not only on the lang and xml:lang attributes,
but also on the HTTP Content-Language header.

This may be problematic for JavaScript browser implementations since there is no current
way to get the Content-Language header of a DOM Document object.  That means those 
implementations will not be fully conforming, unless the rule is changed so as to rely
on only the lang and xml:lang attributes, or a new DOM attribute is added that retrieves
the Content-Language header, or in some other way.

Moreover, I don't believe this requirement is fully reflected in the current
test suite, I'm not sure.

--Peter