The popup message which you are referring is the outcome of Constraint API's element.setCustomValidity() method.

Note: HTML5 Constraint validation doesn't remove the need for validation on the server side. Even though far fewer invalid form requests are to be expected, invalid ones can still be sent by non-compliant browsers (for instance, browsers without HTML5 and without JavaScript) or by bad guys trying to trick your web application. Therefore, like with HTML4, you need to also validate input constraints on the server side, in a way that is consistent with what is done on the client side.

Conclusion

As the constraint validation is designed to work of checking the form content on the client side those elements won't be available in the DOM Tree.