Div嵌套防止穿透事件

<div style="width:100px; height:100px; border:1px solid #000" id="x">
<div style="width:50px; height:50px; margin:0 auto; border:1px solid #ccc" id="y"></div>
</div>
<script type="text/javascript">
document.getElementById("x").onclick=function(e)
{
    alert("x")
    e = e || window.event;
　　if (e.stopPropagation )
　　      e.stopPropagation();
　　else
　      　e.cancelBubble = true;


}
document.getElementById("y").onclick=function(e)
{
    alert("y")
   e = e || window.event;
　　if (e.stopPropagation )
　　      e.stopPropagation();
　　else
　      　e.cancelBubble = true;
}
</script>