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diff --git a/layout/reftests/web-animations/stacking-context-opacity-losing-css-animation-in-delay.html b/layout/reftests/web-animations/stacking-context-opacity-losing-css-animation-in-delay.html
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+++ b/layout/reftests/web-animations/stacking-context-opacity-losing-css-animation-in-delay.html
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+<!DOCTYPE html>
+<title>
+CSS opacity animation winning over web animation in delay phase creates
+a stacking context
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+@keyframes Opacity1 {
+  from, to { opacity: 1; }
+}
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+  animation: Opacity1 100s;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+  document.getElementById("test")
+    .animate({ opacity: [0.1, 0.1] }, { duration: 100000, delay: 100000 });
+
+  requestAnimationFrame(function() {
+    document.documentElement.classList.remove("reftest-wait");
+  });
+</script>