Dead Center

This will show up in the vertical and horizontal center regardless of screen size.

#deadcenter {
    width: 600px;
    height: 300px;
    position: absolute;
    top: 50%;
    left: 50%;
    margin: -150px 0 0 -300px;
}

See the Pen Dead Center by ninjajalapeno (@ninjajalapeno) on CodePen.