知识点：

1、辅助类
2、响应式工具
3、字体图标
4、下拉菜单

1、辅助类

1）文本

  <p class="text-muted">本行内容是减弱的</p>
  <p class="text-primary">本行内容带有一个 primary class</p>
  <p class="text-success">本行内容带有一个 success class</p>
  <p class="text-info">本行内容带有一个 info class</p>
  <p class="text-warning">本行内容带有一个 warning class</p>
  <p class="text-danger">本行内容带有一个 danger class</p>

2）背景

  <p class="bg-primary">bootstrap课程</p>
  <p class="bg-success">bootstrap课程</p>
  <p class="bg-info">bootstrap课程</p>
  <p class="bg-warning">bootstrap课程</p>
  <p class="bg-danger">bootstrap课程</p>

3）关闭按钮 close

  <button class="close">×</button>

4）下拉式菜单 caret

  <span class="caret"></span>

5）浮动 pull-left pull-right

  <div class="pull-left">向左快速浮动</div>
  <div class="pull-right">向右快速浮动</div>

6）清除浮动 clearfix

  <div class="clearfix"  style="background: #D8D8D8;border: 1px solid #000;padding: 10px;">
     <div class="pull-left" style="background:#58D3F7;">向左快速浮动</div>
     <div class="pull-right" style="background: #DA81F5;">向右快速浮动</div>
  </div>

7）块级内容居中 center-block

  <div class="row">
     <div class="center-block" style="width:200px;background-color:#ccc;">
        这是 center-block
     </div>
  </div>

8）显示、隐藏 show hide

  <div class="row" style="padding: 91px 100px 19px 50px;">
     <div class="show" style="width:300px;background-color:#ccc;">
        这是 show class
     </div>
     <div class="hidden" style="width:200px;background-color:#ccc;">
        这是 hide class
     </div>
  </div>

9）屏幕阅读器和键盘导航 .sr-only 来把元素对所有设备隐藏，除了屏幕阅读器 .sr-only-focusable

  <div class="row" style="padding: 91px 100px 19px 50px;">
     <form class="form-inline" role="form">
         <div class="form-group">
            <label class="sr-only" for="email">Email 地址</label>
            <input type="email" class="form-control" placeholder="Enter email">
         </div>
         <div class="form-group">
            <label class="sr-only" for="pass">密码</label>
            <input type="password" class="form-control" placeholder="Password">
         </div>
     </form>
  </div>
  <a class="sr-only sr-only-focusable" href="#content">Skip to main content</a>

2、响应式工具

1）visible-xs visible-sm visible-md visible-lg

  hidden-xs   hidden-sm   hidden-md   hidden-lg
  以超小屏幕（xs）为例，可用的 .visible-*-* 类是：visible-xs-block、visible-xs-inline 和 visible-xs-inline-block。

2）visible-print-block visible-print-inline visible-print-inline-block 浏览器隐藏 打印机可见

  hidden-print  浏览器可见   打印机隐藏
  visible-print

 <div class="container" style="padding: 40px;">
  <div class="row">
     <div class="col-xs-6 col-sm-3" style="background-color: #dedef8;border:1px solid #000;">
        <span class="hidden-xs">特别小型</span>
        <span class="visible-xs">✔ 在特别小型设备上可见</span>
     </div>
     <div class="col-xs-6 col-sm-3" style="background-color: #dedef8;border:1px solid #000;">
        <span class="hidden-sm">小型</span>
        <span class="visible-sm">✔ 在小型设备上可见</span>
     </div>
     <div class="col-xs-6 col-sm-3" style="background-color: #dedef8;border:1px solid #000;">
        <span class="hidden-md">中型</span>
        <span class="visible-md">✔ 在中型设备上可见</span>
     </div>
     <div class="col-xs-6 col-sm-3" style="background-color: #dedef8;border:1px solid #000;">
        <span class="hidden-lg">大型</span>
        <span class="visible-lg">✔ 在大型设备上可见</span>
     </div>
  </div>
 </div>

3、字体图标

用法：<span class="glyphicon glyphicon-search"></span>

4、下拉菜单

1）基本的下拉菜单

  <div class="dropdown">
     <button class="btn btn-default dropdown-toggle" data-toggle="dropdown">Dropdown <span class="caret"></span></button>
     <ul class="dropdown-menu">
        <li><a href="">Html</a></li>
        <li><a href="">Javascript</a></li>
        <li><a href="">jQuery</a></li>
        <li><a href="">html5+css3</a></li>
     </ul>
  </div>

2）对齐

  dropdown-menu-right

3）标题 不要加超链接

  <ul class="dropdown-menu">
     ...
     <li class="dropdown-header">Dropdown header</li>
     ...
  </ul>

4）分隔线

  <ul class="dropdown-menu">
     ...
     <li class="divider"></li>
     ...
  </ul>

5）禁用的菜单项

  <ul class="dropdown-menu" aria-labelledby="dropdownMenu4">
     <li><a href="#">Regular link</a></li>
     <li class="disabled"><a href="#">Disabled link</a></li>
     <li><a href="#">Another link</a></li>
  </ul>

6）让菜单默认显示

  <div class="dropdown open">