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diff --git a/layout/reftests/web-animations/stacking-context-opacity-changing-effect.html b/layout/reftests/web-animations/stacking-context-opacity-changing-effect.html
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index 0000000000..6d5324e132
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+++ b/layout/reftests/web-animations/stacking-context-opacity-changing-effect.html
@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<html class="reftest-wait reftest-no-flush">
+<title>
+  Opacity animation creates a stacking context after changing effects
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+div {
+  width: 100px;
+  height: 100px;
+  background: blue;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+  var elem = document.getElementById("test");
+  var anim = elem.animate(null, { duration: 100000 });
+  var newEffect = new KeyframeEffect(elem, { opacity: [1, 1] }, 100000);
+  anim.ready.then(function() {
+    anim.effect = newEffect;
+    requestAnimationFrame(function() {
+      document.documentElement.classList.remove("reftest-wait");
+    });
+  });
+</script>
+</html>