The serverside resource is called via jQuery.ajax (XMLHttpRequest) and gets a key/value pair corresponding to the name of the validated element and its value as a GET parameter. The serverside response must be a JSON string that must be
for valid elements, and can be
for invalid elements, using the default error message. If the serverside response is a string, eg.
"That name is already taken, try peter123 instead"
, this string will be displayed as a custom error message in place of the default.
For more examples, take a look the marketo demo and the milk demo.