xml:lang="" vs HTTP

Assertion holds if you see green below.

This test is only valid if the user agent recognises the HTTP and html xml:lang declarations, and :lang works.

Ignore this test. (:lang not supported) 

Assertion: If the HTTP header contains a language declaration but the html element uses an empty xml:lang attribute, the UA will not recognize the language declared in the HTTP header.

Notes:
Red means the HTTP had precedence.

Next test
xhtml as xml