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diff --git a/layout/reftests/css-animations/stacking-context-opacity-removing-important-in-delay.html b/layout/reftests/css-animations/stacking-context-opacity-removing-important-in-delay.html
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+++ b/layout/reftests/css-animations/stacking-context-opacity-removing-important-in-delay.html
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+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>
+Removing !important rule during delay phase of animation creates
+a stack context for correct style
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+@keyframes Opacity0 {
+  from, to { opacity: 0 }
+}
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+  animation: Opacity0 100s 100s;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+window.addEventListener("load", () => {
+  var target = document.getElementById("test");
+  target.style.setProperty("opacity", "0.1", "important");
+
+  requestAnimationFrame(() => {
+    // Now the target opacity style should be 0.1 because of !important rule.
+
+    // Apply 100% opacity without important directive.
+    target.style.setProperty("opacity", "1", "");
+    requestAnimationFrame(() => {
+      // The CSS animation is no longer overridden but it's still in delay
+      // phase, so we should create a stacking context for 100% opacity style.
+      document.documentElement.classList.remove("reftest-wait");
+    });
+  });
+});
+</script>