allineare div al centro

[miki003]

<style type="text/css">
<!--
#layer1 {
position: absolute;
visibility: visible;
width: 400px;
height: 300px;
left: 50%;
top: 50%;
background-color: #ccc;
border: 1px solid #000;
margin: -150px 0 0 -200px;
padding: 0px;
}
-->
</style>