When using the
z-index attribute as a property in script, please remember to capitalize the "I" as in "zIndex"
Also ensure that the objects whose zIndex you are trying to change have been positioned absolutely. In the example below please note the use of "position:relative" which makes the "divRel" bounding box the starting point for the left, right. top and bottom
attributes of the absolutely positioned child elements. (In the images below, the styling of the buttons at the bottom has been modified by my Windowblinds skin.)

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
<title>test</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<style type="text/css">
<!--
body {width:100%;background:#999;color:#000;text-align:left;margin:0;padding:20px;font:bold 14px/18px Arial;}
#divRel {width:300px;height:300px;margin:0 auto;background:#777;position:relative;}
#box1, #box2 {width:240px;height:240px;position:absolute;text-align:center;}
#box1 {left:20px;top:20px;background-color:#ff0;z-index:2;}
#box2 {right:20px;bottom:20px;background-color:#0f0;z-index:1;}
#inputBar {width:300px;margin:10px auto 0;text-align:center;}
input {width:140px;}
p {margin-top:110px;}
-->
</style>
<script type="text/javascript">
function fx(){
document.getElementById("yOnTop").onclick=function(){document.getElementById("box2").style.zIndex=1;};
document.getElementById("gOnTop").onclick=function(){document.getElementById("box2").style.zIndex=3;};
}
</script>
</head>
<body onload="fx()">
<div id="divRel">
<div id="box1"><p>Yellow div is on top</p></div>
<div id="box2"><p>Green div is on top</p></div>
</div>
<div id="inputBar">
<input type="button" id="yOnTop" value="Yellow" title="Make the yellow div topmost"/>
<input type="button" id="gOnTop" value="Green" title="Make the green div topmost"/>
</div>
</body>
</html>