[form] HTML - <optgroup> 태그 - 선택사항묶음 (= optgroup태그 = 옵트그룹태그, 옵션그룹태그)

 
<select>
  <optgroup label="community">
    <option value="greeting">가입인사</option>
    <option value="free">자유게시판</option>
  </optgroup>
  <optgroup label="homzzang">
    <option value="html">HTML</option>
    <option value="css">CSS</option>
  </optgroup>
</select>