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diff --git a/testing/web-platform/tests/css/css-contain/container-queries/container-units-shadow.html b/testing/web-platform/tests/css/css-contain/container-queries/container-units-shadow.html
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index 0000000000..1a711dad00
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+++ b/testing/web-platform/tests/css/css-contain/container-queries/container-units-shadow.html
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+<!doctype html>
+<title>Container Relative Units: Shadow DOM</title>
+<link rel="help" href="https://drafts.csswg.org/css-contain-3/#container-lengths">
+<link rel="help" href="https://drafts.csswg.org/css-contain-3/#container-queries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/resources/declarative-shadow-dom-polyfill.js"></script>
+<script src="support/cq-testcommon.js"></script>
+<style>
+  #outer {
+    container-type: size;
+    width: 200px;
+    height: 200px;
+  }
+  #direct {
+    container-type: inline-size;
+    width: 50cqw;
+    height: 50cqh;
+  }
+  #nondirect {
+    width: 10cqw;
+    height: 10cqh;
+    background: green;
+  }
+</style>
+<div id=outer>
+  <div>
+    <template shadowroot="open">
+      <style>
+        #inner {
+          container-type: size;
+          width: 30px;
+          height: 30px;
+        }
+      </style>
+      <div id=inner>
+        <slot></slot>
+      </div>
+    </template>
+    <div>
+      <div id=direct>
+        <div id=nondirect>
+        </div>
+      </div>
+    </div>
+  </div>
+</div>
+<script>
+  setup(() => {
+    assert_implements_container_queries();
+    polyfill_declarative_shadow_dom(document);
+  });
+
+  test(() => {
+    let cs = getComputedStyle(direct);
+    assert_equals(cs.width, '100px');
+    assert_equals(cs.height, '100px');
+  }, 'Direct slotted child queries shadow-including ancestors');
+
+  test(() => {
+    let cs = getComputedStyle(nondirect);
+    assert_equals(cs.width, '10px'); // #direct
+    assert_equals(cs.height, '20px'); // #outer
+  }, 'Nondirect slotted child queries shadow-including ancestors');
+</script>