build/dom4.max.js
function initCustomEvent(
        type, bubbles, cancelable, detail
      ) {
        /*jshint validthis:true*/
        this.initEvent(type, bubbles, cancelable);
        this.detail = detail;
      }
function initCustomEvent(
        type, bubbles, cancelable, detail
      ) {
        /*jshint validthis:true*/
        this.initEvent(type, bubbles, cancelable);
        this.detail = detail;
      }
attached at runtime