* Specifies something to do when an element matching a CSS selector is encountered.
* @param {String} selector a CSS selector triggering your behavior
* @param {String} id combined with selector, uniquely identifies this behavior; prevents duplicate registrations
* @param {Number} priority relative position of this behavior in case multiple apply to a given element; lower numbers applied first (sorted by id then selector in case of tie); choose 0 if you do not care
* @param {Function} behavior callback function taking one parameter, a (DOM) {@link Element}, and returning void