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diff --git a/testing/web-platform/tests/web-animations/animation-model/keyframe-effects/background-shorthand.html b/testing/web-platform/tests/web-animations/animation-model/keyframe-effects/background-shorthand.html
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+++ b/testing/web-platform/tests/web-animations/animation-model/keyframe-effects/background-shorthand.html
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+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>CSS Animations: Expansion of shorthand properties</title>
+<link rel="help" href="https://www.w3.org/TR/web-animations-1/#calculating-computed-keyframes">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #block {
+    background: green;
+    background-position-x: 10px;
+    height: 100px;
+    width: 100px;
+  }
+</style>
+<body>
+  <div id="block"></div>
+</body>
+<script>
+  function assert_background_position_at(time, x, y) {
+    document.getAnimations()[0].currentTime = time;
+    const target = document.getElementById('block');
+    const style = getComputedStyle(target);
+    assert_equals(style.backgroundPositionX, x,
+                  `background-position-x @${time/10}% progress`);
+    assert_equals(style.backgroundPositionY, y,
+                  `background-position-y @${time/10}% progress`);
+  }
+
+  test(() => {
+    const target = document.getElementById('block');
+    target.animate([
+      { background: 'red' },
+      { backgroundPositionY: '10px', background: 'blue' }
+    ], { duration: 1000 });
+    // Animation is active in the semi-closed interval [0, 1000).
+    // The background shorthand expands to its longhand counterparts with
+    // background-position-(x|y) picking up the default value of 0%.
+    // The explicit background-property-y in the second keyframe takes priority
+    // over the value from expansion of the shorthand.
+    const test_cases = [
+      { time: -100, x: "10px", y: "0%" },
+      { time: 0, x: "0%", y: "0%" },
+      { time: 200, x: "0%", y: "calc(0% + 2px)" },
+      { time: 500, x: "0%", y: "calc(0% + 5px)" },
+      { time: 800, x: "0%", y: "calc(0% + 8px)" },
+      { time: 1100, x: "10px", y: "0%" }
+    ];
+    test_cases.forEach(test => {
+      assert_background_position_at(test.time, test.x, test.y);
+    });
+  }, 'Shorthand properties expand to longhand counterparts in computed ' +
+     'keyframes.');
+</script>