position: absolute;
left: 50%;
top: 50%;
margin-left: -150px;
margin-top: -150px;
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
margin: auto;
position: absolute;
left: 50%;
top: 50%;
margin-left: -150px;
margin-top: -150px;
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
margin: auto;