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diff --git a/layout/reftests/web-animations/style-updates-on-iteration-composition-changed-from-replace-to-accumulate.html b/layout/reftests/web-animations/style-updates-on-iteration-composition-changed-from-replace-to-accumulate.html
new file mode 100644
index 0000000000..bf4d40b259
--- /dev/null
+++ b/layout/reftests/web-animations/style-updates-on-iteration-composition-changed-from-replace-to-accumulate.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Update styles when iteration composition changed from replace to
+accumulate</title>
+<script src="animation-utils.js"></script>
+<style>
+#test {
+  width: 100px; height: 100px;
+  background: blue;
+}
+</style>
+<div id="test"></div>
+<script>
+  var anim = document.getElementById("test")
+    .animate({ marginLeft: [ "0px", "100px" ] },
+             { duration: 100000,
+               delay: -99999, // For starting right before second iteration.
+               easing: "steps(1, start)",
+               iterations: 2,
+               iterationComposite: "replace" });
+
+  waitForIterationChange(anim).then(() => {
+    // Changing iterationComposite updates the element style.
+    anim.effect.iterationComposite = "accumulate";
+    // Now margin-left of the element should be 200px.
+    document.documentElement.classList.remove("reftest-wait");
+  });
+</script>