The HTML5 draft says, "The lang attribute (in no namespace) specifies the primary language for the element's contents and for any of the element's attributes that contain text. Its value must be a valid BCP 47 language code, or the empty string."

http://dev.w3.org/html5/spec/dom.html#the-lang-and-xml:lang-attributes

RFC 5646 (BCP 47) says: "Although valid in language tags, subtags and tags with a 'Deprecated' field are deprecated, and validating processors SHOULD NOT generate these subtags."

http://tools.ietf.org/html/rfc5646#page-28

So "grandfathered" tags that are deprecated are still valid BCP 47 language codes. Given that, it would seem that validator.nu should report them as Warnings instead of as Errors.