Use setCustomValidity:
document.addEventListener("DOMContentLoaded", function() {
var elements = document.getElementsByTagName("INPUT");
for (var i = 0; i < elements.length; i++) {
elements[i].oninvalid = function(e) {
e.target.setCustomValidity("");
if (!e.target.validity.valid) {
e.target.setCustomValidity("This field cannot be left blank");
}
};
elements[i].oninput = function(e) {
e.target.setCustomValidity("");
};
}
})I changed to vanilla JavaScript from Mootools as suggested by @itpastorn in the comments, but you should be able to work out the Mootools equivalent if necessary.
Edit
I've updated the code here as setCustomValidity works slightly differently to what I understood when I originally answered. If setCustomValidity is set to anything other than the empty string it will cause the field to be considered invalid; therefore you must clear it before testing validity, you can't just set it and forget.
Further edit
As pointed out in @thomasvdb's comment below, you need to clear the custom validity in some event outside of invalid otherwise there may be an extra pass through the oninvalid handler to clear it.