UL LI结构实现二级导航菜单(HTML+CSS+JS)

UL LI结构的二级导航下拉菜单，HTML+CSS+JS来实现。

修改说明：

宽高主要是修改下面的几个样式。

#nav a {
 display: block; width: 80px; text-align:center;
}
#nav li {
 float: left; width: 80px; background:#CCC;
}
#nav li ul li{
 float: left; width: 180px;
 background: #F6F6F6;
}
#nav li ul a{
 display: block; width: 165px;_width: 180px;height:auto; margin:0 auto;text-align:left; padding-left:15px;
}

部分结构如下：

<ul id="nav">
<li><a href="http://www.51xuediannao.com">懒人建站</a>
    <ul>
        <li><a href="http://www.51xuediannao.com/JS/nav/">CSS导航菜单</a></li>
        <li><a href="http://www.51xuediannao.com/JS/Slide/">FLASH焦点图</a></li>
        <li><a href="http://www.51xuediannao.com/JS/texiao/">网页特效JS</a></li>
        <li><a href="http://www.51xuediannao.com/JS/AD/">JS广告代码</a></li>
    </ul>
</li>
<li><a href="http://www.51xuediannao.com/show">欣赏借鉴</a>
     <ul>
        <li><a href="http://www.51xuediannao.com/show/qiye/">企业类网站</a></li>
        <li><a href="http://www.51xuediannao.com/show/hangye/">行业类网站</a></li>
        <li><a href="http://www.51xuediannao.com/show/blog/">BLOG类网站</a></li>
        <li><a href="http://www.51xuediannao.com/show/menhu/">门户类网站</a></li>
        <li><a href="http://www.51xuediannao.com/show/shop/">商城类网站</a></li>
     </ul>
</li>
<li><a href="http://www.51xuediannao.com/jiqiao/">建站技巧</a>
     <ul>
        <li><a href="http://www.51xuediannao.com/jiqiao/html/">HTML技巧</a></li>
        <li><a href="http://www.51xuediannao.com/jiqiao/css/">CSS技巧</a></li>
        <li><a href="http://www.51xuediannao.com/jiqiao/cms/">CMS建站技巧</a></li>
        <li><a href="http://www.51xuediannao.com/jiqiao/qita/">其他技巧</a></li>
     </ul>
</li>
<li><a href="http://51xuediannao.com">懒人建站</a>
    <ul>
        <li><a href="http://www.51xuediannao.com/JS/nav/">CSS导航菜单</a></li>
        <li><a href="http://www.51xuediannao.com/JS/Slide/">FLASH焦点图</a></li>
        <li><a href="http://www.51xuediannao.com/JS/texiao/">网页特效JS</a></li>
        <li><a href="http://www.51xuediannao.com/JS/AD/">JS广告代码</a></li>
    </ul>
</li>

<li><a href="#">在线演示</a>
    <ul>
        <li><a href="http://51xuediannao.com/JS/nav/">CSS导航菜单</a></li>
        <li><a href="http://51xuediannao.com/JS/Slide/">FLASH焦点图</a></li>
        <li><a href="http://51xuediannao.com/JS/texiao/">网页特效JS</a></li>
        <li><a href="http://51xuediannao.com/JS/AD/">JS广告代码</a></li>
    </ul>
</li>
<li><a href="http://51xuediannao.com/jiqiao/">建站技巧</a>
     <ul>
        <li><a href="http://51xuediannao.com/jiqiao/html/">HTML技巧</a></li>
        <li><a href="http://51xuediannao.com/jiqiao/css/">CSS技巧</a></li>
        <li><a href="http://51xuediannao.com/jiqiao/cms/">CMS建站技巧</a></li>
        <li><a href="http://51xuediannao.com/jiqiao/qita/">其他技巧</a></li>
     </ul>
</li>
</ul>