sgml-validate-command

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

"Install (o)nsgmls, tidy, or some other SGML validator, and set `sgml-validate-command'"

• This variable may be risky if used as a file-local variable.

Documentation:
The command to validate an SGML document.
The file name of current buffer file name will be appended to this,
separated by a space.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.