mozilla-release/layout/reftests/css-transitions/stacking-context-transform-wins-over-important-style.html

<!DOCTYPE html>
<html class="reftest-wait">
<title>
Transform transition winning over !important rule creates a stacking context
</title>
<style>
span {
  height: 100px;
  width: 100px;
  position: fixed;
  background: green;
  top: 50px;
}
#test {
  width: 100px; height: 100px;
  background: blue;
  /*
   * On the compositor we use a previous vsync time stamp rather than the
   * current time stamp, as a result sometimes transition may be still in
   * before phase after waiting a frame. To compose the same transform value
   * regardless of whether the transition is in before or active phase, we use
   * steps(1, end) here.
   */
  transition: transform 100s steps(1, end);
  transform: none ! important;
}
</style>
<span></span>
<div id="test"></div>
<script>
window.addEventListener("load", () => {
  var target = document.getElementById("test");
  getComputedStyle(target).transform;

  target.style.setProperty("transform", "translateX(200px)", "important");
  getComputedStyle(target).transform;

  document.documentElement.classList.remove("reftest-wait");
});
</script>