UXP/layout/reftests/position-dynamic-changes/vertical/style.css

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