gecko-dev/gfx/layers/apz/test/helper_subframe_style.css

body {
  height: 500px;
}

.inner-frame {
  margin-top: 25%;
  height: 200%;
  width: 75%;
  overflow: scroll;
}
.inner-content {
  height: 200%;
  width: 200%;
  background: repeating-linear-gradient(#EEE, #EEE 100px, #DDD 100px, #DDD 200px);
}