Bootstrap记录

左侧 导航下拉：

 <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">
                      <i class="fa fa-list"></i> Forms <b class="fa fa-plus dropdown-plus"></b>
                    </a>
                    <ul class="dropdown-menu">
                      <li>
                        <a href="form-elements.html">
                          <i class="fa fa-caret-right"></i> Common Elements
                        </a>
                      </li>
                      <li>
                        <a href="validation-elements.html">
                          <i class="fa fa-caret-right"></i> Validation
                        </a>
                      </li>
                      <li>
                        <a href="form-wizard.html">
                          <i class="fa fa-caret-right"></i> Form Wizard
                        </a>
                      </li>
                    </ul>
                  </li>