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diff --git a/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog.html b/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog.html
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index 0000000000..9d18b1862d
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+++ b/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog.html
@@ -0,0 +1,45 @@
+<!doctype html>
+<title>CSS Container Queries Test: @container with modal dialog child</title>
+<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="support/cq-testcommon.js"></script>
+<style>
+  #container {
+    container-type: inline-size;
+  }
+  dialog {
+    color: red;
+  }
+  @container (max-width: 200px) {
+    dialog { color: green; }
+  }
+  @container (max-width: 100px) {
+    dialog { color: lime; }
+  }
+</style>
+<div id="container">
+  <dialog id="dialog"></dialog>
+</div>
+<script>
+  setup(() => assert_implements_container_queries());
+
+  test(() => {
+    assert_equals(getComputedStyle(dialog).color, "rgb(255, 0, 0)");
+  }, "#container initially wider than 200px");
+
+  test(() => {
+    container.style.width = "200px";
+    assert_equals(getComputedStyle(dialog).color, "rgb(0, 128, 0)");
+  }, "#container changed to 200px");
+
+  test(() => {
+    dialog.showModal();
+    assert_equals(getComputedStyle(dialog).color, "rgb(0, 128, 0)");
+  }, "Modal dialog still has parent as query container while in top layer");
+
+  test(() => {
+    container.style.width = "100px";
+    assert_equals(getComputedStyle(dialog).color, "rgb(0, 255, 0)");
+  }, "Container changes width while dialog is in top layer");
+</script>