palemoon-dev/platform/layout/reftests/z-index/stacking-context-common.css

.contain, .maybesc {
  width: 100px;
  height: 100px;
}
.contain, .maybesc {
  position: relative;
}
.inner1, .inner3, .intruder {
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}
.inner1 {
  left: 5px; right: 30px;
  background: rgba(0, 255, 255, 0.5); /* aqua */
  z-index: 1;
}
.inner3 {
  left: 30px; right: 5px;
  background: rgba(255, 255, 0, 0.5); /* yellow */
  z-index: 3;
}
.intruder {
  top: 20px; bottom: 20px;
  background: rgba(255, 0, 255, 0.5); /* fuchsia */
  z-index: 2;
}