<!DOCTYPE html>

<html>
    <head>
        <title>form1</title>
    </head>
    <body>
        <form action="/register" id="registration" method="get">
            <input name="email" placeholder="Email" type="text"/>
            <input name="password" placeholder="Password" type="password"/>
            <input name="confirmation" placeholder="Password (again)" type="password"/>
            <input name="agreement" type="checkbox"/> I agree
            <input type="submit" value="Register"/>
        </form>
        <script>

            var form = document.getElementById('registration');
            form.onsubmit = function() {

                if (form.email.value == '')
                {
                    alert('missing email');
                    return false;
                }
                else if (form.password.value == '')
                {
                    alert('missing password');
                    return false;
                }
                else if (form.password.value != form.confirmation.value)
                {
                    alert('passwords don\'t match');
                    return false;
                }
                else if (!form.agreement.checked)
                {
                    alert('checkbox unchecked');
                    return false;
                }
                return true;

            };

        </script>
   </body>
</html>