Conformance checkers must use the information given on the WHATWG Wiki MetaExtensions page to establish if a value not explicitly defined in this specification is allowed or not. -- http://www.whatwg.org/html5#other-metadata-names
I think we should allow conformance checkers which can be run on local files without an internet connection, but the above appears to deem them non-conforming. The general conformance requirements include this get-out clause, but it isn't broad enough to cover this case: User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations. -- http://www.whatwg.org/html5#conformance-requirements Smylers