This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
As noted in Bug #14020, the W3C HTML5 validatom service has support for HTML5+Microdata (though it labels it as pure HTML5 - which is wrong). The Www-validator@ mailing list quite often gets questions about how to add the the Facebook open graph namespace. If HTML5+RDFa validaton were available, it would be possible to a) use the FB namespace, as well as the HTML+RDFa's @prefix attribute in order to declare the prefix. The validator is scewed towards Microdata as long as it does not offer RDFa, yet continues to stamp Microdata as valid.
The Nu Markup Validation Service supports both HTML5 + RDFa Lite 1.1 (by default) and HTML5 + RDF Core 1.1 (as an option).