> How do you notify a user of errors in input without a complete page refresh if you do not use Javascript (ignoring the most basic HTML stuff)?

Well, it used to be that submitting a form and rendering errors on the server took the same ~100ms that evaluating a javascript framework validation does now, and so it was... not a problem.

If you mean "leaving aside the most trivial html stuff" to include things like html5 inputs, that's the most significant part of what Javascript provides in form validation.