In commit 5ca3378452140283b75eec95063dd1ac439661b0 the xhtml namespace was to
use https rather than http. While the URL does resolve with a 200 OK over
https, the namespace is still defined in the xhtml 1 and 1.1 specs to be http.
Since the current namespace setting fails markup validation, attached patch
reverts back to http.
cheers ./daniel