When I submit the form I got an alert message. When I accept the alert it will submit the form anyway. Returning false is ignored.
Onclick can not be used. I try with var x = document.forms[form][fname].value;
and still same.

<form id=f method=post name=form onsubmit=return validateForm(); action=#>

    <input type=text name=fname id=test />

    <input type=submit value=submit/>

</form>

<script type=text/javascript>

        function validateForm() {

            var x = document.getElementById('test').value;

            if (x == null || x == 0 || x == 0) {

                alert(Stop);

                return false;

            }

        }

    </script>