Many common content types are registered at the IANA . LanguageCode Attribute values of type LanguageCode specify a language code according to RFC 1766 . Examples of language codes include en for English, en-US for American English, and ja for Japanese. Whitespace is not allowed in the language code, which is case-insensitive. These attribute values specify character encodings of linked resources.
Character encodings are case-insensitive.
A list of registered character encodings is available at the IANA . Character Character attribute values take a single Unicode character. The character may be specified with an entity . Note that the T separating the date from the time must appear literally and is case-sensitive. LinkTypes Attribute values of type LinkTypes give a space-separated list of link types.