Ho un layout che mi sta facendi uscire pazzo:

In particiolare non riesco a capire perche' in ie ho un risultato diverso rispetto a firefox, visto che lo stile e' cosi' semplice e non uso stranezze....

<!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">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<script type="text/javascript" src="./js/jquery-1.5.2.js"></script>
<style type="text/css">
body
{
font-family: "Lucida Grande", Tahoma, Arial, Verdana, sans-serif;
font-size: small;
}

#contenitore
{
border: 1px solid red;
width: 950px;
height: 600px;
padding: 1%;
text-align: left;
}

#login_div
{
border: 1px solid red;
background-color: #C7E19E;
width: 400px;
height: 300px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -200px;
margin-left: -200px;
padding: 1%;
text-align: left;
}

#form_div
{
border: 1px solid red;
width: 200px;
height: 200px;
}

#messaggi_div
{
border: 1px solid red;
width: 400px;
height: 50px;
}

.form
{
border: 1px solid red;
margin: 25px 15px 0 0;
}

.form label
{
border: 1px solid red;
font-size: 13px;
}

.form span
{
border: 1px solid blue;
display: block;
float: left;
margin: 0 0 10px 10px;
}

.form input
{
border: 1px solid red;
display: block;
}

</style>

</head>
<body>
<center>
<div id="login_div">
<h2>Autenticazione</h2>
<h4>Inserire le crenziali di accesso</h4>
<hr>
<form id="login_form" class = "form" method="post">
<span>
<input id = "username" type="text" name="username">
<label for = "username">Utente</label>
</span>
<span>
<input id = "password" type="password" name="password">
<label for = "password">Password</label>
</span>
<input id="save_form" class="button_text" type="submit" name="submit" value="Accedi" />
</form>
<div id="messaggi_div">
</div">
</div>
</center>
</body>
</html>