<select name="empty"></select>
<select name="empty-break-line">
</select>
<select name="empty-new-line">
  
</select>
<select name="default">
  <option value="1">select</option>
</select>

select {
  border-color: red;
  border-width: 5px;
}

select:empty {
  border-color: green;
}

select:not(:has(option)) {
  border-color: blue;
}