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diff --git a/layout/reftests/css-animations/stacking-context-lose-opacity-1.html b/layout/reftests/css-animations/stacking-context-lose-opacity-1.html
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+++ b/layout/reftests/css-animations/stacking-context-lose-opacity-1.html
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+<!DOCTYPE html>
+<title>
+Opacity animation creates a stacking context even if the opacity property
+is overridden by an !important rule
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+@keyframes Opaque {
+  from, to { opacity: 1 }
+}
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+  animation: Opaque 100s;
+  opacity: 1 !important;
+}
+</style>
+<span></span>
+<div id="test"></div>