sgml-xml-mode

sgml-xml-mode is a variable defined in `sgml-mode.el'.
Its value is

nil

Documentation:
When non-nil, tag insertion functions will be XML-compliant.
It is set to be buffer-local when the file has
a DOCTYPE or an XML declaration.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.