The prefix xml is by definition bound to the namespace name http://www.w3.org/XML/1998/namespace. It may, but need not, be declared, and must not be undeclared or bound to any other namespace name. Other prefixes must not be bound to this namespace name, and it must not be declared as the default namespace.
I ran into this with the commonly used xml:lang attribute without explicit definition of the xml prefix.
https://www.w3.org/TR/xml-names/#ns-decl
I ran into this with the commonly used xml:lang attribute without explicit definition of the xml prefix.