HTML5 form required attribute. Set custom validation message?

It's very simple to control custom messages with the help of HTML5 event oninvalid

Here is code:

<input id="UserID"  type="text" required="required"
       oninvalid="this.setCustomValidity('Witinnovation')"
       onvalid="this.setCustomValidity('')">

This is most important:

onvalid="this.setCustomValidity('')"

Here is the code to handle custom error message in HTML5:

<input type="text" id="username" required placeholder="Enter Name"
  oninvalid="this.setCustomValidity('Enter User Name Here')"
  oninput="this.setCustomValidity('')"/>

This part is important because it hides the error message when the user inputs new data:

oninput="this.setCustomValidity('')"

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.