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diff --git a/layout/reftests/css-animations/stacking-context-opacity-wins-over-transition.html b/layout/reftests/css-animations/stacking-context-opacity-wins-over-transition.html
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+++ b/layout/reftests/css-animations/stacking-context-opacity-wins-over-transition.html
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+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>
+Opacity animation winning over opacity transition creates a stacking context
+for correct style.
+</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;
+  opacity: 1;
+  transition: opacity 100s steps(1, start);
+  animation: Opacity1 100s;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+window.addEventListener("load", () => {
+  var target = document.getElementById("test");
+  getComputedStyle(target).opacity;
+
+  // CSS animation wins over transitions, so transition won't be visible during
+  // the CSS animation.
+  target.style.opacity = 0;
+  requestAnimationFrame(() => {
+    document.documentElement.classList.remove("reftest-wait");
+  });
+});
+</script>