#outer {
  position: absolute;
  top: 50%;
  left: 0px;
  width: 100%;
  height: 1px;
  overflow: visible;
}

#inner {
  width: 300px;
  height: 200px;
  margin-left: -150px;  /***  width / 2   ***/
  position: absolute;
  top: -100px;          /***  height / 2   ***/
  left: 50%;
  text-align: center;
}