マウスが指定された領域コードにあるかどうかを判断します.

<div style="width:300px; height:300px; border:1px solid red;">
　　<div id="target"><div><span> </span></div></div>
</div>

<style type="text/css">
#target{ width: 200px; height: 200px; background: #EEE;}
#target div{width: 150px; height: 150px; border:1px solid orange;}
#target span{ display: block; width: 50px; height: 50px; background: gray;}
</style>

var target = document.getElementById('target');
	if(target.addEventListener){
		target.addEventListener("mouseout",mouseoutHander,false)
	}else if(target.attachEvent){
		target.attachEvent("onmouseout",mouseoutHander);
	}else{
		target.onmouseout = mouseoutHander;
	}
	function mouseoutHander(e){
		e = e || window.event;
		var target = e.target || e.srcElement;

		  //  
		  if (target.id !== 'target') {
		    return;
		  }

		  //  
		  var relatedTarget = e.relatedTarget || e.toElement;
		  while (relatedTarget !== target
		    && relatedTarget.nodeName.toUpperCase() !== 'BODY') {
		    relatedTarget = relatedTarget.parentNode;
		  }

		  //  ， 
		  if (relatedTarget === target) {
		    return;
		  }

		  //  
		  alert(' ');
	}