#auto {
    width:      400px;
    height:     300px;
    border: 1px solid #0000ff;
    margin: auto;
}

#center {
    width:      400px;
    height:     300px;
    position:   absolute;
    top:        50%;
    left:       50%;
    margin-left:    -200px;
    margin-top:     -150px;
    border: 1px solid #00ff00;
}