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diff --git a/testing/web-platform/tests/css/css-masking/clip-path/animations/clip-path-animation-three-keyframes1.html b/testing/web-platform/tests/css/css-masking/clip-path/animations/clip-path-animation-three-keyframes1.html
new file mode 100644
index 0000000000..61e24e6208
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+++ b/testing/web-platform/tests/css/css-masking/clip-path/animations/clip-path-animation-three-keyframes1.html
@@ -0,0 +1,45 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<link rel="help" href="https://drafts.csswg.org/css-shapes-1/#basic-shape-interpolation">
+<link rel="match" href="clip-path-animation-three-keyframes1-ref.html">
+<style>
+.container {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+  /* Use a long animation that start at 5% progress where the slope of the
+     selected timing function is zero. By setting up the animation in this way,
+     we accommodate lengthy delays in running the test without a potential drift
+     in the animated property value. This is important for avoiding flakes,
+     especially on debug builds. The screenshots are taken as soon as the
+     animation is ready, thus the long animation duration has no bearing on
+     the actual duration of the test. */
+  animation: clippath 1000000s cubic-bezier(0,1,1,0) -50000s;
+}
+@keyframes clippath {
+  0% { clip-path: ellipse(10% 20% at 50% 50%) }
+  10% {
+    clip-path: ellipse(20% 30% at 70% 70%);
+    animation-timing-function: cubic-bezier(0,1,1,0);
+  }
+  100% {
+    clip-path: ellipse(10% 20% at 50% 50%);
+    animation-timing-function: cubic-bezier(0,1,1,0);
+  }
+}
+</style>
+<script src="/common/reftest-wait.js"></script>
+<body>
+<div class="container"></div>
+
+<script>
+// This test ensures that we select the correct start and end keyframes for
+// interpolation. In this test, the start delay of the animation makes it
+// jump to 5% right away. So for this test, we would choose the keyframes at
+// 0% and 10% for interpolation.
+document.getAnimations()[0].ready.then(() => {
+  takeScreenshot();
+});
+</script>
+</body>
+</html>