#overlay_dhtml {
  display: block;
  position: fixed;
  z-index: 10;
  width: 100%;
  height: 100%;
  background-image: url('/overlay.png');
  background-repeat: repeat;
}
#cadre_dhtml {
  z-index: 20;
  width:500px;
  height:250px;
  position: fixed;
  margin-top:100px;
  position:relative;
}