2.1.1 [XML] Section 2.3, Common Syntactic Constructs
[Definition: A Name is a token beginning with a letter or one of a few punctuation characters, and continuing with letters, digits, hyphens, underscores, colons, or full stops, together known as name characters.] Names beginning with the string "xml", or with any string which would match (('X'|'x') ('M'|'m') ('L'|'l')), are reserved for standardization in this or future versions of this specification.
MSXML3 and MSXML6
Any name that begins with a string that matches (('X'|'x') ('M'|'m') ('L'|'l')) is valid and not defined as reserved.