HTML+CSS竖向折叠点击展开的两级菜单导航

HTML+CSS结构的竖向折叠菜单，点击展开二级菜单，结构为HTML+CSS结构，清晰明了，易修改。

废话少说上结构：

<div id="my_menu" class="sdmenu">
      <div>
        <span>JS代码</span>
        <a href="http://51xuediannao.com/JS/nav/">CSS导航</a>
        <a href="http://51xuediannao.com/JS/Slide/">焦点图片</a>
        <a href="http://51xuediannao.com/JS/AD/">JS广告代码</a>
        <a href="http://51xuediannao.com/JS/texiao/">网页特效JS</a>
      </div>
  
      <div>
        <span>欣赏借鉴</span>
        <a href="http://51xuediannao.com/show/qiye/">企业类网站</a>
        <a href="http://51xuediannao.com/show/blog/">博客类网站</a>
        <a href="http://51xuediannao.com/show/shop/">商城类网站</a>
      </div>
      <div>
        <span>建站技巧</span>
        <a href="http://51xuediannao.com/jiqiao/html/">HTML技巧</a>
        <a href="http://51xuediannao.com/jiqiao/css/">CSS技巧</a>
        <a href="http://51xuediannao.com/jiqiao/cms/">CMS技巧</a>
       </div>
       <div>
        <span>懒人建站</span>
        <a href="http://51xuediannao.com/HTML+CSS/">HTML+CSS模板</a>
        <a href="http://51xuediannao.com/SEO/">SEO优化</a>
       </div>
    </div>