<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<script type="text/javacsript">
function showMenu(thisa) {
thisa.style.height = "180px";
}
function hideMenu(thisa) {
thisa.style.height = "30px";
}
</script>
<div onmouseover='showMenu(this)' onmouseout='hideMenu(this)' style='width:120px;height:30px;border:1px solid blue;overflow:hidden;background:#cccccc'>
<table>
<tr>
<td>下拉菜单</td>
</tr>
<tr>
<td>菜单一</td>
</tr>
<tr>
<td>菜单二</td>
</tr>
<tr>
<td>菜单三</td>
</tr>
<tr>
<td>菜单四</td>
</tr>
<tr>
<td>菜单五</td>
</tr>
</table>
</div>
</body>
</html>