Bootstrap下拉头部

<div class = dropdown>

   <button type = button class = btn dropdown-toggle id = dropdownMenu1 data-toggle = dropdown>
      Topics
      <span class = caret></span>
   </button>

   <ul class = dropdown-menu role = menu aria-labelledby = dropdownMenu1>
      <li role = presentation class = dropdown-header>Dropdown header</li>

      <li role = presentation >
         <a role = menuitem tabindex = -1 href = #>Java</a>
      </li>

      <li role = presentation>
         <a role = menuitem tabindex = -1 href = #>Data Mining</a>
      </li>

      <li role = presentation>
         <a role = menuitem tabindex = -1 href = #>
            Data Communication/Networking
         </a>
      </li>

      <li role = presentation class = divider></li>
      <li role = presentation class = dropdown-header>Dropdown header</li>

      <li role = presentation>
         <a role = menuitem tabindex = -1 href = #>Separated link</a>
      </li>
   </ul>

</div>