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diff --git a/testing/web-platform/tests/html/semantics/forms/the-form-element/form-indexed-element-shadow.html b/testing/web-platform/tests/html/semantics/forms/the-form-element/form-indexed-element-shadow.html
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+++ b/testing/web-platform/tests/html/semantics/forms/the-form-element/form-indexed-element-shadow.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>form.elements: indexed access reflects DOM order, not flat tree</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<form id="target">
+  <div id="host">
+    <template shadowrootmode="open">
+      <slot name="first"></slot>
+      <slot name="second"></slot>
+    </template>
+    <input id="first" slot="second">
+    <input id="second" slot="first">
+  </div>
+</form>
+<script>
+test(function() {
+  let target = document.getElementById("target");
+  let host = document.getElementById("host");
+  assert_true(!!host.shadowRoot, "Should have a shadow tree");
+  assert_equals(target.elements[0], first, "form.elements reflects DOM order, not flat tree order");
+  assert_equals(target.elements[1], second);
+});
+</script>