The xmlns
attribute is deprecated in HTML+RDFa 1.1.
You should use the prefix
attribute instead:
<html
prefix="og: http://ogp.me/ns#
fb: http://www.facebook.com/2008/fbml">
resp. if you want to keep the xmlns
for XHTML5:
<html
xmlns="http://www.w3.org/1999/xhtml"
prefix="og: http://ogp.me/ns#
fb: http://www.facebook.com/2008/fbml">