gecko/modules/plugin/test/reftest/plugin-background.css

div {
  position: absolute;
}
#bad {
  left:220px; top:0px;
  z-index: 0;
}
#good {
  left:0px; top:0px;
  width:220px; height:220px;
  background-color: rgba(0,255,0, 0.6);
  z-index: 0;
}

#topbar {
  left:0px; top:0px;
  width:220px; height:20px;
  background-color: rgb(0,0,0);
  z-index: 2;
}
#topbar {
  left:0px; top:0px;
  width:220px; height:20px;
  background-color: rgb(0,0,0);
  z-index: 2;
}
#leftbar {
  left:0px; top:0px;
  width:20px; height:220px;
  background-color: rgb(0,0,0);
  z-index: 2;
}
#rightbar {
  left:200px; top:0px;
  width:20px; height:220px;
  background-color: rgb(0,0,0);
  z-index: 2;
}
#bottombar {
  left:0px; top:200px;
  width:220px; height:20px;
  background-color: rgb(0,0,0);
  z-index: 2;
}

div#plugin {
  position: absolute;
  left:1px; top:1px;
  width:199px; height:199px;
  background-color: rgba(0,0,255, 0.2);
  z-index: 1;
}

embed#plugin {
  position: absolute;
  left:1px; top:1px;
  z-index: 1;
}