Why is xmlns attribute mandatory on html element ?

Hello,

I am surprised that the validator claims that xmlns should be mandatory on the
html element of xhtml documents.

As far as I know, xmlns is declared as #FIXED.
I always interpreted a #FIXED attribute as not being mandatory in the  
source xml document, but having to be returned in any case by the  
parser (being present or not). Is this wrong ?

Does it mean that #FIXED attributes MUST appear in the source document ?
or is xmlns a special case ?

Any answer would be appreciated.

Regards,
Daniel