纯css实现，鼠标划过显示二级菜单。二级菜单自上至下滑入。
<!doctype html>
<html class="no-js" lang="en">
<head>
<meta charset="utf-8">
<title>纯CSS3-红色下拉导航菜单代码</title>
<style>
/*导航栏*/
.nav_menu3,.nav_menu3 ul,.nav_menu3 ul li,.nav_menu3 ul li a,.nav_menu3 #menu-button {margin: 0;padding: 0;border: 0;list-style: none;line-height: 1;display: block;position: relative;-webkit-box-sizing: border-box;-moz-box-sizing: border-box;box-sizing: border-box;}
.nav_menu3 #menu-button { margin-top: 100px; background: #F00; position: absolute; z-index: 99999;}
.nav_menu3:after,.nav_menu3 > ul:after {content: ".";display: block;clear: both;visibility: hidden;line-height: 0;height: 0;}
.nav_menu3 #menu-button {display: none;}
.nav_menu3 {width: auto;line-height: 1;background: #ffffff;background: #990B2C;}
#menu-line {position: absolute;top: 0;left: 0;height: 3px;background: #990B2C;-webkit-transition: all 0.25s ease-out;-moz-transition: all 0.25s ease-out;-ms-transition: all 0.25s ease-out;-o-transition: all 0.25s ease-out;transition: all 0.25s ease-out;}
.nav_menu3 > ul > li {float: left;}
.nav_menu3.align-center > ul {font-size: 0;text-align: center;}
.nav_menu3.align-center > ul > li {display: inline-block;float: none;}
.nav_menu3.align-center ul ul {text-align: left;}
.nav_menu3.align-right > ul > li {float: right;}
.nav_menu3.align-right ul ul {text-align: right;}
.nav_menu3 > ul > li > a {padding: 30px 10px 29px 10px;text-decoration: none;text-transform: uppercase;color: #eee;-webkit-transition: color .2s ease;-moz-transition: color .2s ease;-ms-transition: color .2s ease;-o-transition: color .2s ease;transition: color .2s ease;border-right:1px solid #990B2C;}
.nav_menu3 > ul > li:hover > a {color: #990B2C;background: #FFF;}
.nav_menu3 > ul > li.nav-has-sub > a {padding-right: 25px;}
.nav_menu3 > ul > li.nav-has-sub > a::after {position: absolute;top: 35px;right: 10px;width: 4px;height: 4px;border-bottom: 1px solid #990B2C;border-right: 1px solid #990B2C;content: "";-webkit-transform: rotate(45deg);-moz-transform: rotate(45deg);-ms-transform: rotate(45deg);-o-transform: rotate(45deg);transform: rotate(45deg);-webkit-transition: border-color 0.2s ease;-moz-transition: border-color 0.2s ease;-ms-transition: border-color 0.2s ease;-o-transition: border-color 0.2s ease;transition: border-color 0.2s ease;}
.nav_menu3 > ul > li.nav-has-sub:hover > a::after {border-color: #990B2C;}
.nav_menu3 ul ul {position: absolute;left: -9999px;}
.nav_menu3 > ul > li > ul { border-top: 1px solid #FFF;}
.nav_menu3 li:hover > ul {left: auto;}
.nav_menu3.align-right li:hover > ul {right: 0;}
.nav_menu3 ul ul ul {margin-left: 100%;top: 0;}
.nav_menu3.align-right ul ul ul {margin-left: 0;margin-right: 100%;}
.nav_menu3 ul ul li {height: 0;-webkit-transition: height .2s ease;-moz-transition: height .2s ease;-ms-transition: height .2s ease;-o-transition: height .2s ease;transition: height .2s ease;}
.nav_menu3 ul li:hover > ul > li {height: 52px;}
.nav_menu3 ul ul li a {padding: 20px 20px;width: 140px;background: #990B2C;text-decoration: none;color: #eeeeee;-webkit-transition: color .2s ease;-moz-transition: color .2s ease;-ms-transition: color .2s ease;-o-transition: color .2s ease;transition: color .2s ease;border-top: 1px solid #990B2C;}
.nav_menu3 ul ul li:hover > a,.nav_menu3 ul ul li a:hover {color: #990B2C;background: #FFF;border-top: 1px solid #FFF;}
.nav_menu3 ul ul li.nav-has-sub > a::after {position: absolute;top: 25px;right: 10px;width: 4px;height: 4px;border-bottom: 1px solid #eee;border-right: 1px solid #eee;content: "";-webkit-transform: rotate(-45deg);-moz-transform: rotate(-45deg);-ms-transform: rotate(-45deg);-o-transform: rotate(-45deg);transform: rotate(-45deg);-webkit-transition: border-color 0.2s ease;-moz-transition: border-color 0.2s ease;-ms-transition: border-color 0.2s ease;-o-transition: border-color 0.2s ease;transition: border-color 0.2s ease;}
.nav_menu3.align-right ul ul li.nav-has-sub > a::after {right: auto;left: 10px;border-bottom: 0;border-right: 0;border-top: 1px solid #dddddd;border-left: 1px solid #dddddd;}
.nav_menu3 ul ul li.nav-has-sub:hover > a::after {border-color: #990B2C;}
</style>
</head>
<body>
<div class="nav_menu3">
<ul>
<li class='nav-has-sub'><a href='index.html'>庄严禅院</a>
<ul>
<li><a href='#'>缘起</a></li>
<li><a href='#'>寺院建设过程</a></li>
<li><a href='#'>远景</a></li>
<li><a href='#'>功德榜芳名</a></li>
<li><a href='#'>出入账明细表</a></li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>智觉流通</a>
<ul>
<li><a href='#'>各类经书</a></li>
<li><a href='#'>戒律</a></li>
<li><a href='#'>藏</a></li>
<li class='nav-has-sub'><a href='#'>论</a>
<ul>
<li><a href='#'>问答</a></li>
<li><a href='#'>解惑</a></li>
</ul>
</li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>中国传统文化</a>
<ul>
<li><a href='#'>中医</a></li>
<li><a href='#'>礼仪</a></li>
<li><a href='#'>黄帝内经</a></li>
<li><a href='#'>咨询</a></li>
<li><a href='#'>免费养老</a></li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>世界历学</a>
<ul>
<li><a href='#'>世界各地文化</a></li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>人类交流</a>
<ul>
<li><a href='#'>企业管理</a></li>
<li><a href='#'>商业模式咨询</a></li>
<li><a href='#'>科技咨询</a></li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>游戏人间</a>
<ul>
<li><a href='#'>佛教电影/电视</a></li>
<li class='nav-has-sub'><a href='#'>游戏</a>
<ul>
<li><a href='#'>共建极乐</a></li>
<li><a href='#'>各种游戏</a></li>
<li><a href='#'>猜迷语/猜字</a></li>
</ul>
</li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>人间杂会</a>
<ul>
<li><a href='#'>八卦</a></li>
<li><a href='#'>新闻</a></li>
<li><a href='#'>社会</a></li>
<li><a href='#'>易经</a></li>
<li><a href='#'>小说/故事</a></li>
</ul>
</li>
<li class='nav-has-sub'><a href='#'>智力学习</a>
<ul>
<li><a href='#'>小学</a></li>
<li><a href='#'>初中</a></li>
<li><a href='#'>高中</a></li>
</ul>
</li>
</ul>
</div>
<div style="text-align:center;margin:50px 0; font:normal 14px/24px 'MicroSoft YaHei';">
<p>适用浏览器：360、FireFox、Chrome、Safari、Opera、傲游、搜狗、世界之窗. 不支持IE8及以下浏览器。</p>
</div>
</body>
</html>