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

.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;
}