게시판에 해당되는 것은 아니고..간단한 소개 페이지에 2단메뉴를 만들려는데요(코드첨부)

<head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><script type="text/javascript"><!--//글로벌 네비게이션(2Depth 메뉴그룹)에 대한 마우스 또는 키보드 반응(보임/숨김)설정function activeGNB(id) {for(num=1; num<=3; num++) document.getElementById('gm'+num).style.display='none'; //D2MG1~D2MG4 까지 숨긴 다음document.getElementById(id).style.display='block'; //해당 ID만 보임}//--></script><style type="text/css">* { margin:0; padding:0; border:0; }ul { list-style:none }a {color:#FFFFFF; text-decoration:none }a:hover,a:focus,a:active { color:#FFFF00; text-decoration:none; font-weight:bold }/* 레이아웃 */#gnb { position:relative;  margin-bottom:10px; }/* 글로벌 메뉴 배치 */#gnb:after { content:""; display:block; clear:both; }#gnb li { float:left; text-align:center; background-color:silver; border:1px solid black;}#gnb li li { float:left; width:auto; text-align:left; }#gnb li a { display:block; height:20px;}#gnb li li a { display:inline; height:auto; padding:0; margin:0 10px; }#gnb ul { position:relative;}#gnb ul ul { position:relative; top:38px; display:none; white-space:nowrap; }#gnb ul#gm1 { left:0; }#gnb ul#gm2 { left:25%; }#gnb ul#gm3 { left:50%; }</style><!--[if IE]><style type="text/css">#body,#gnb,#gnb li a,#gnb ul{ zoom:1;}#gnb ul ul{ _width:100%;}</style><![endif]--></head><body><div id="gnb"><ul> <li class="g1"><a href="#" OnMouseOver="activeGNB('gm1');" onfocus="activeGNB('gm1');">about us</a>  <ul id="gm1">    <li><a href="#">회사개요</a></li>    <li><a href="#">찾아오시는길</a></li>  </ul>  </li> <li class="g1"><a href="#" OnMouseOver="activeGNB('gm2');" onfocus="activeGNB('gm2');">business</a>  <ul id="gm2">   <li><a href="#">사업분야</a></li>   <li><a href="#">GIS</a></li>   <li><a href="#">ESL</a></li>   <li><a href="#">SMARTPHONE</a></li>  </ul> </li> <li class="g1"><a href="#" OnMouseOver="activeGNB('gm3');" onfocus="activeGNB('gm3');">contact us</a>  <ul id="gm3">   <li><a href="#">공지사항</a></li>   <li><a href="#">게시판</a></li>  </ul> </li></ul></div></body>