Consider what exactly constitutes an XML document, according to the W3C's Recommendation for XML 1.0:
A data object is an XML document if it is well-formed, as defined in this specification. A well-formed XML document may in addition be valid if it meets certain further constraints.
This definition indicates that the minimum requirement for a document to be considered an XML document is that it be well-formed.

Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. It is popular among compiler writers and programming language researchers, as well as in the development of theorem provers. SML is a modern descendant of the ML programming language used in the LCF theorem-proving project. It is uncommon among widely used languages in that it has a formal specification, given as typing rules and operational semantics in The Definition of Standard ML (1990, revised and simplified as The Definition of Standard ML (Revised) in 1997).