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diff --git a/layout/reftests/web-animations/stacking-context-opacity-changing-keyframe-in-delay.html b/layout/reftests/web-animations/stacking-context-opacity-changing-keyframe-in-delay.html
new file mode 100644
index 0000000000..e0864f1d58
--- /dev/null
+++ b/layout/reftests/web-animations/stacking-context-opacity-changing-keyframe-in-delay.html
@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>
+Changing keyframes to opacity frames creates a stacking context even if the
+animation is delay phase
+</title>
+<style>
+span {
+  height: 100px;
+  width: 100px;
+  position: fixed;
+  background: green;
+  top: 50px;
+}
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+}
+</style>
+<span></span>
+<div id="test"></div>
+<script>
+  var anim = document.getElementById("test")
+    .animate({ width: ['100px', '100px'] },
+             { delay: 100000, duration: 100000 });
+  anim.ready.then(function() {
+    anim.effect.setKeyframes({ opacity: [0, 1] });
+    document.documentElement.classList.remove("reftest-wait");
+  });
+</script>