The version number of
the specification. This attribute is intended for processing systems to
determine the appropriate versions of Schema, Schematron, and CVE values for
validation and interpretation of an instance document.
If there are multiple of
this attribute specified in an instance document, the first one in document
order is the one that will apply to the entire document. Document order is
defined in https://www.w3.org/TR/xpath-datamodel/#document-order