select-appearance-picker-select-border-ref.html

<!DOCTYPE html>
<link rel=stylesheet href="resources/stylable-select-styles.css">

<div id=container class=stylable-select-container>
  <button class="stylable-select-button open" popovertarget=popover id=button>
    <span class=stylable-select-selectedoption>option</span>
    <div class=stylable-select-button-icon></div>
  </button>
  <div id=popover popover=auto anchor=container class=stylable-select-datalist>
    <div class="stylable-select-option selected">option</div>
  </div>
</div>

<style>
.stylable-select-datalist {
  border: 5px solid red;
}
</style>

<script>
document.getElementById('popover').showPopover();
</script>

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