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+++ b/layout/reftests/css-animations/stacking-context-opacity-win-in-delay-on-main-thread.html
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+<!DOCTYPE html>
+<title>
+Opacity animation winning over another opacity animation in delay phase
+on the main-thread creates a stacking context
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+@keyframes Opacity1 {
+  from, to { opacity: 1; }
+}
+@keyframes Opacity0 {
+  from, to { opacity: 0; }
+}
+// For preventing running on the compositor.
+@keyframes Width {
+  from, to { width: 100px; }
+}
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+  animation: Opacity0 100s 100s, Opacity1 100s, Width 100s;
+}
+</style>
+<span></span>
+<div id="test"></div>