body, html {
height: 100%;
width: 100%;
margin: 0px;
padding: 0px;
}

#center {
padding:0px;
border:0px;

width:800px;
height:600px;

margin-left:-400px; /* -((Width / 2) - padding - border) */
margin-top:-300px;  /* -((Height / 2) - padding - border) */

position:absolute;
left:50%;
top:50%;
}