> Why is the namespace not correct? 
because it's got the wrong characters in it.
the xhtml namespace is
http://www.w3.org/1999/xhtml
and you declared (but didn't use)
http://www.w3.org/1999/XHTML
which is a different namespace.

> I just tested the link...
Whether a server serves anything from a URI has no relationship to
whether that URI is usable as a namespace for a particular vocabulary.
Thw w3c server  rewrites the URI to lowercase if it would otherwise
fail a GET request which is a convenience if you are trying to get web
pages, but irrelevant to namespace processing which is case sensitive.

David

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