2.2.6 [XML] Section 3, Logical Structures

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C0006:

The specification states:

 This specification does not constrain the application semantics, use, or (beyond 
 syntax) names of the element types and attributes, except that names beginning with 
 a match to (('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 is not defined as reserved.