动态改变div 的z-index属性的问题，[请帮忙改一下程序，谢谢！]

Question

<script language="javascript">
var MAX_INDEX=4;
function change(ddd)
{
if(ddd.style.zIndex<= MAX_INDEX)
{
ddd.style.zIndex = MAX_INDEX+1;
MAX_INDEX++;
}
}
</script>
<div id="layer1" onclick="javascript:change(this);" style="position:absolute;margin-left:100;margin-top=100;width=100;height=100;background-color:#EEEEEE;z-index:1"></div>
<div id="layer2" onclick="javascript:change(this);" style="position:absolute;margin-left:120;margin-top=120;width=100;height=100;background-color:#DDDDDD;z-index:2"></div>
<div id="layer3" onclick="javascript:change(this);" style="position:absolute;margin-left:140;margin-top=140;width=100;height=100;background-color:#CCCCCC;z-index:3"></div>
<div id="layer4" onclick="javascript:change(this);" style="position:absolute;margin-left:160;margin-top=160;width=100;height=100;background-color:#AAAAAA;z-index:4"></div>