Possible Bug in Tidy

hello Dave Raggett,
i have attached a copy of a test html in which
Tidy warns me about the attrib. xml:lang="en" in the HTML tag. 
According to this Page { http://www.w3.org/TR/2000/REC-xhtml1-20000126/#normative } 
section 3.1.1 in the example provided (in green), this is completely fine.. is it a bug?

thankx 
Rodrigo

==
--------BEGIN GEEK CODE BLOCK--------------
Version: 3.12
GG d++ s:- a-- C++++ UL p+ L++ E --- W++ N+ o+ K- w
O- M V PS PE+ X PGP- T 5 X R tv- b+ DI+ 
D G e h r y+
--------END GEEK CODE BLOCK----------------

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