denotes an attribute whose value is a language code for the natural language of the content of any element; its value is inherited. This name is reserved by virtue of its definition in the XML specification.
Attempting to install the relevant ISO 2- and 3-letter codes as the enumerated possible values is probably never going to be a realistic possibility.
See BCP 47 at http://www.rfc-editor.org/rfc/bcp/bcp47.txt and the IANA language subtag registry at http://www.iana.org/assignments/language-subtag-registry for further information.
The union allows for the 'un-declaration' of xml:lang with the empty string.
denotes an attribute whose value is a keyword indicating what whitespace processing discipline is intended for the content of the element; its value is inherited. This name is reserved by virtue of its definition in the XML specification.
denotes an attribute whose value provides a URI to be used as the base for interpreting any relative URIs in the scope of the element on which it appears; its value is inherited. This name is reserved by virtue of its definition in the XML Base specification.
See http://www.w3.org/TR/xmlbase/ for information about this attribute.
denotes an attribute whose value should be interpreted as if declared to be of type ID. This name is reserved by virtue of its definition in the xml:id specification.
See http://www.w3.org/TR/xml-id/ for information about this attribute.