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diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html
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index 0000000000..742c35b258
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+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-in-container-query.html
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+<!DOCTYPE html>
+<title>scroll-timeline and container queries</title>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#scroll-timeline-shorthand">
+<link rel="help" src="https://drafts.csswg.org/css-contain-3/#container-queries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<script src="support/testcommon.js"></script>
+<style>
+  #outer {
+    height: 100px;
+    width: 150px;
+  }
+
+  #container {
+    container-type: size;
+  }
+
+  #scroller {
+    overflow: auto;
+    width: auto;
+    height: 100px;
+  }
+
+  #scroller > div {
+    height: 200px;
+  }
+
+  /* This does not apply initially. */
+  @container (width > 200px) {
+    #scroller {
+      scroll-timeline: --timeline;
+    }
+  }
+
+  @keyframes recolor {
+    from { background-color: rgb(100, 100, 100); }
+    to { background-color: rgb(200, 200, 200); }
+  }
+
+  #element {
+    height: 10px;
+    width: 10px;
+    animation: recolor 10s linear;
+    animation-timeline: --timeline;
+    background-color: rgb(0, 0, 0);
+  }
+</style>
+<div id=outer>
+  <div id=container>
+    <div id=scroller>
+      <div></div>
+      <div id=element></div>
+    </div>
+  </div>
+</div>
+<script>
+  setup(assert_implements_animation_timeline);
+
+  promise_test(async (t) => {
+    element.offsetTop;
+    scroller.scrollTop = (scroller.scrollHeight - scroller.clientHeight) / 2;
+    await waitForNextFrame();
+    // Unknown timeline, time held at zero.
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(100, 100, 100)');
+    // This causes the timeline to be created.
+    outer.style.width = '250px';
+    // Check value with getComputedStyle immediately, which is the unanimated
+    // value since the scroll timeline is inactive before the next frame.
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(0, 0, 0)');
+    // Also check value after one frame.
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).backgroundColor, 'rgb(150, 150, 150)');
+  }, 'Timeline appearing via container queries');
+</script>