纯JavaScript实现点击向下展开的下拉菜单

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>下拉菜单</title>
<style type="text/css">
*{margin:0; padding:0}
#nav{width:200px; margin:50px}
#nav h3{ cursor:pointer; line-height:30px; height:30px; background-color:#000000; color:#fff}
#nav a{display:block; line-height:24px;color:#666666}
#nav a:hover{background-color:#eee; color:#000;}
#nav div{display:none; border:1px solid #000; border-top:none}
</style>
<script type="text/javascript">
function $(id){return document.getElementById(id)}
window.onload = function(){
 $("nav").onclick = function(e){
 var src = e?e.target:event.srcElement;
 if(src.tagName == "H3"){
  var next = src.nextElementSibling || src.nextSibling;
  next.style.display = (next.style.display =="block")?"none":"block";
 }
 }
}
</script>
</head>
<body>
<div id="nav">
 <h3>管理区</h3>
  <div>
   <a href="#">建议</a>
    <a href="#">链接</a>
    <a href="#">联系</a>
  </div>
  <h3>交流区</h3>
  <div>
   <a href="#">JavaScript</a>
    <a href="#">Delphi</a>
    <a href="#">VC++</a>
  </div>
</div>
</body>
</html>