Quick validation

How could I validate forms even before pressing submit button using ajax ?. Error must appear just after entered the value in to the field.
Eg:

 

To validate without clicking the submit button you can use something like:

    var el = document.getElementById('inputFieldId');

    el.onblur = function() {

        var value_to_validate = el.value;

        //make an ajax request and send the value from the input field for validation

        //if the request returns an error attach the proper message to the proper place in the DOM

    }

Code (markup):

You can follow this tutorial on how to make ajax requests to validate a form:

http://www.designchemical.com/blog/index.php/jquery/create-your-own-jquery-ajax-form-submit-with-validation/