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diff --git a/layout/reftests/css-transitions/stacking-context-opacity-wins-over-important-style.html b/layout/reftests/css-transitions/stacking-context-opacity-wins-over-important-style.html
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index 000000000..c167df077
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+++ b/layout/reftests/css-transitions/stacking-context-opacity-wins-over-important-style.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>
+Opacity 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 opacity value
+   * regardless of whether the transition is in before or active phase, we use
+   * steps(1, end) here.
+   */
+  transition: opacity 100s steps(1, end);
+  opacity: 1 ! important;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+window.addEventListener("load", () => {
+  var target = document.getElementById("test");
+  getComputedStyle(target).opacity;
+
+  target.style.setProperty("opacity", "0", "important");
+  getComputedStyle(target).opacity;
+
+  requestAnimationFrame(() => {
+    document.documentElement.classList.remove("reftest-wait");
+  });
+});
+</script>