#container
{
    height: 400px;
    border: 1px solid black;
    position: relative;
}
#box
{
    height: 80px;
    background-color: limegreen;
    border: 2px solid forestgreen;
    border-radius: 5px;
    width: 80px;
    height: 80px;
    text-align: center;
    color: white;
    position: absolute;
}