How to use WTForms in Ajax validation?
A great question. This is what we do (flask backend, jquery frontend):

use jquery.forms plugin to ajax forms. Pretty solid mature code. Shortcoming, cannot send json encoded data, only form-urlencoded. Receives plain or json data. 
use wtfroms for form validation. Very mature codebase.
tried to use wtforms-json for accepting json, very screwy problems.
I accustomed of using WTForms by means of Flask-WTF in my flask application. Doing server side validation is trivial. But how do I leverage this server validation to become a field level, ajax, client side validation? So, when user tab to another input fields, my application can directly goes on validating it and give validation warning/info/error.
I haven't found a resource in the internet yet