The current HTML5 draft says that a document's character encoding declaration "must be an ASCII case-insensitive match for the preferred MIME name for that encoding" and that "the preferred MIME name of a character encoding is the name or alias labeled as "preferred MIME name" in the IANA Character Sets registry, if there is one, or the encoding's name, if none of the aliases are so labeled."

http://dev.w3.org/html5/spec/semantics.html#charset
http://dev.w3.org/html5/spec/infrastructure.html#preferred-mime-name

The current v.nu "charset" and "meta-charset" datatype checking does not consult the IANA Character Sets registry at all. It seems that what it optimally should do is something similar to the way in which the "language" datatype checking relies on parsing and consulting the IANA Language Subtag registry.

http://www.iana.org/assignments/character-sets