#outerDiv{
width: 500px;
height: 500px;
position:relative;
background-color: lightgrey;
}
#innerDiv{
width: 284px;
height: 290px;
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
-ms-transform: translate(-50%, -50%); /* IE 9 */
-webkit-transform: translate(-50%, -50%); /* Chrome, Safari, Opera */
background-color: grey;
}