Non capisco perchè il seguente menù a tendina mi funziona con mozilla ma non con explorer! Forse ho sbagliato qualcosa nel codice?

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Menu</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

<!--[if lt IE 7]>
<link rel="stylesheet" href="/IE7/ie7-html.css" type="text/css"/>
<![endif]-->
<style type="text/css">
div#hormenu {
width: 150px; 
float: left; 
margin: 0; 
padding: 0;
background: #ffffff;
font-family: Arial, Helvetica, sans-serif; font-size: 10px; color:#333333;
}
div#hormenu ul {
margin: 0;
padding: 0;
list-style-type: none;
}
div#hormenu li  {
width:150px;
float:left;
margin: 0;
padding: 0;
}
div#hormenu a {
display: block;
padding: 3px;
color: #000000;
text-decoration: none;
}
div#hormenu a:hover { 
background-color: #FFFFFF;
color: #CC0000;
text-decoration: none;
}
div#hormenu li ul {display: none;}
div#hormenu li:hover ul {
position: absolute;
display: block;
z-index:1;
width:120px;
padding: 0px;
margin: -20px 0 0 80px;
border:1px solid black;
background: white;
}
div#hormenu li li {border: none; width: 150px;}	
div#hormenu li li a {padding: 2px 2px 2px 10px;}
</style>
</head>
<body>
<div id="hormenu"> 
<ul>[*]HOME[*]LO STUDIO[*]SERVIZI
<ul>[*]circolari[*]modulistica[*]deduzione figli[/list]
[*]CONTATTI[*]AVVERTENZE[/list]
</div>
</body>
</html>