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chromium/third_party/blink/web_tests/external/wpt/shadow-dom/focus/focus-tab-on-shadow-host.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>HTML Test: Use tab to navigate the focus to an element inside shadow host with delegatesFocus</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>
<script src="resources/shadow-utils.js"></script>

<body>
  <div id="host"></div>
</body>

<script>
const host = document.getElementById("host");

const shadowRoot = host.attachShadow({ mode: "open", delegatesFocus: true });
const input = document.createElement("input");
shadowRoot.appendChild(input);

promise_test(async function() {
  assert_equals(document.activeElement, document.body);
  // Press <tab>
  await navigateFocusForward();
  assert_equals(document.activeElement, host);
  assert_equals(shadowRoot.activeElement, input);
  assert_true(host.matches(':focus'));
  assert_true(input.matches(':focus'));
  assert_true(input.matches(':focus-visible'));
}, ":focus should be applied to the host and :focus-visible should be applied to the child node when the focus is moved by <tab>");
</script>