<form>

 <label>Password: <input type="password" id="password1" oninput="checkPasswords()"></label>

 <p>

 <label>Repeat password: <input type="password" id="password2" oninput="checkPasswords()"></label>

 <p>

 <input type="submit"/>

</form>

​

<script> 

 function checkPasswords() {

 var password1 = document.getElementById('password1');

 var password2 = document.getElementById('password2');

 if (password1.value != password2.value) {

 password2.setCustomValidity('Passwords non identiques');

 } else {

 password2.setCustomValidity('');

 }

 }

</script>