the form speaking to the handler is almost done I guess. Atm it is running in debug mode talking to test.wikimedia.de. I pulled the subscriptions branch on the test wiki, so the request handling works and even sends emails. I handle errors from the ajax.php iterating over field value pairs given in the "errors" parameter.
You can test it here:
The code for the JS, CSS and proxy is here:
The HTML for the featurebox is here: