gecko-dev/layout/reftests/position-dynamic-changes/horizontal/style.css

#parent {
  position: relative;
  width: 400px;
  height: 400px;
  background: red;
  border: 0 solid black;
}
#child {
  position: absolute;
  height: 100px;
  top: 100px;
  background: blue;
  border: 0 solid black;
}
.floatLeft {
  width: 50px;
  height: 10px;
  background: green;
  float: left;
}