div.center
{   width:200px;
    height:100px;
    border: thin solid black;
    position: relative;
    margin: auto;
    background-color: white;
}

div.center div#ab
{   position: absolute;
    left: 50px;
    top:  25px;
    width: 50px;
    height: 50px;
    background-color: darkblue;
}