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chromium/third_party/blink/web_tests/external/wpt/html/semantics/forms/the-selectlist-element/selectlist-overflow-x-ref.tentative.html 

<!DOCTYPE html>
<style>
selectlist {
  width:30px;
  height:20px;
}
selectlist::part(button) {
  background-color:blue;
}
</style>
<selectlist>
  <option>&nbsp;</option>
</selectlist>