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diff --git a/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog-container.html b/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog-container.html
new file mode 100644
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+++ b/testing/web-platform/tests/css/css-contain/container-queries/top-layer-dialog-container.html
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+<!doctype html>
+<title>CSS Container Queries Test: Top layer element as a @container</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>
+  #parent { width: 100px; }
+  #dialog {
+    container-type: inline-size;
+    width: auto;
+    border: none;
+  }
+  #child { color: red; }
+  @container (min-width: 200px) {
+    #child { color: green; }
+  }
+</style>
+<div id="parent">
+  <dialog id="dialog"><span id="child"></span></dialog>
+</div>
+<script>
+  setup(() => assert_implements_container_queries());
+
+  test(() => {
+    assert_equals(getComputedStyle(child).color, "rgb(255, 0, 0)");
+  }, "#dialog initially sized by #containing-block");
+
+  test(() => {
+    dialog.showModal();
+    assert_equals(getComputedStyle(child).color, "rgb(0, 128, 0)");
+  }, "#dialog sized by viewport");
+</script>