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chromium/third_party/blink/web_tests/wpt_internal/html/semantics/forms/the-select-element/stylable-select/select-appearance-picker-select-border.tentative.html 

<!DOCTYPE html>
<html class=reftest-wait>
<link rel=author href="mailto:[email protected]">
<link rel=help href="https://github.com/whatwg/html/issues/9799">
<link rel=match href="select-appearance-picker-select-border-ref.html">
<script src="/resources/testdriver.js"></script>
<script src="/resources/testdriver-vendor.js"></script>

<style>
select {
  appearance: base-select;
}
select::picker(select) {
  border: 5px solid red;
}
</style>

<select>
  <option>option</option>
</select>

<script>
(async () => {
  await test_driver.click(document.querySelector('select'));
  document.documentElement.classList.remove('reftest-wait');
})();
</script>