box width 400px height 200px margin-top -100px margin-left -200px back

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.box {
width: 400px;
height: 200px;
margin-top: -100px;
margin-left: -200px;
background: #000;
border:1px solid #666;
padding:1em;
left: 50%;
top: 50%;
position:absolute;
color: #fff;
text-align: center;
}