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chromium/third_party/blink/web_tests/external/wpt/html/semantics/forms/the-output-element/output-validity.html 

<!DOCTYPE HTML>
<title>:valid and :invalid pseudo-class on output element</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<output id='output_test'></output>

<script>

test(() => {
  let output = document.getElementById("output_test");
  assert_false(output.matches(":valid"), "should not match :valid pseudo-class");
  assert_false(output.matches(":invalid"), "should not match :invalid pseudo-class");

  output.setCustomValidity("custom error");
  assert_equals(output.validationMessage, "", "should not have a validation message");
  assert_true(output.validity.customError, "should have a custom error");
  assert_false(output.validity.valid, "should not be valid with a custom error");
  assert_false(output.matches(":valid"), "should still not match :valid pseudo-class");
  assert_false(output.matches(":invalid"), "should still not match :invalid pseudo-class");
}, ":valid and :invalid pseudo-class on output element")

</script>