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chromium/third_party/blink/web_tests/external/wpt/html/interaction/focus/processing-model/legend-focusable.html 

<!doctype html>
<title>legend focusable</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<script>
 const t = async_test();
</script>
<fieldset>
 <legend tabindex=0 onfocus="t.step(() => { t.done(); })">
  legend
  <input onfocus="t.unreached_func('input in legend was focused')();">
 </legend>
 <input onfocus="t.unreached_func('input after legend was focused')();">
</fieldset>
<script>
  document.querySelector('legend').focus();
</script>