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