[html] Required attribute HTML5

Just put the following below your form. Make sure your input fields are required.

<script>
    var forms = document.getElementsByTagName('form');
    for (var i = 0; i < forms.length; i++) {
        forms[i].noValidate = true;
        forms[i].addEventListener('submit', function(event) {
            if (!event.target.checkValidity()) {
                event.preventDefault();
                alert("Please complete all fields and accept the terms.");
            }
        }, false);
    }
</script>