请问下边的代码为什么在Friefox下不能用呢?
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Testing</title>
<style type="text/css">
#myDiv {
position: absolute;
width: 100px;
height: 100px;
margin-left: expression((document.body.offsetWidth > 500) ? document.body.offsetWidth/2 - 50 + "px" : "200px");
background: red;
}
</style>
</head>
<body>
<div id="myDiv">
</div>
</body>
</html>