I see this on a verbose curl call.
< Content-Type: application/xhtml+xml; charset=utf-8
So I’m not sure that you’re correct in stating that the Content-Type is text/html?
I also ran the site through the w3 validator and it seems to be a bit confused as to whether it is XHTML or HTML. The key thing here though is that the Cards validator will ignore the content if the Content-Type is not one that it recognises.