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Попробуй так:

<mx:HTML id="myhtml" x="0" y="0" width="100%" height="100%" location="http://localhost/test.html" htmlDOMInitialize="makeListeners();"/>
 
private function makeListeners():void {
     var body:Object = myhtml.domWindow.document.body;
 
     body.addEventListener("mousemove", onMouseMove, true);
}
 
private function onMouseMove(evt:MouseEvent):void {
      trace(evt.target.innerHTML);
}