<span class="sr-only">Toggle Dropdown</span>

This is an HTML code for an invisible text that is used for accessibility purposes. The "sr-only" class is used to hide the text visually, but it can still be read by screen readers, allowing users with visual impairments to understand what the element does. In this case, it is likely used for a button or link that toggles something on the webpage.

相关问题

<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">

这是一个用于响应式导航栏的 HTML 元素，它是一个折叠的菜单容器，当用户在小屏幕设备上点击导航栏的按钮时，它会展开或折叠菜单项。其中"class"属性指定了该容器应用的样式，"id"属性指定了该容器的唯一标识符，用于在 JavaScript 中操作该元素。

该元素一般与其他导航栏元素结合使用，例如链接、下拉菜单等，例如：

<nav class="navbar navbar-default">
  <div class="container-fluid">
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand" href="#">Brand</a>
    </div>

    <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
      <ul class="nav navbar-nav">
        <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>
        <li><a href="#">Link</a></li>
        <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
          <ul class="dropdown-menu">
            <li><a href="#">Action</a></li>
            <li><a href="#">Another action</a></li>
            <li><a href="#">Something else here</a></li>
            <li role="separator" class="divider"></li>
            <li><a href="#">Separated link</a></li>
            <li role="separator" class="divider"></li>
            <li><a href="#">One more separated link</a></li>
          </ul>
        </li>
      </ul>
      <form class="navbar-form navbar-right">
        <div class="form-group">
          <input type="text" class="form-control" placeholder="Search">
        </div>
        <button type="submit" class="btn btn-default">Submit</button>
      </form>
    </div>
  </div>
</nav>

在这个例子中，该元素包含了一个无序列表（ul）和一个表单（form），当用户在小屏幕设备上点击导航栏的按钮时，这些元素会展开或折叠。

html 中Bootstrap实现可折叠3级侧边导航菜单样式好看点

可以试试以下代码，它实现了可折叠的3级侧边导航菜单，并且使用了Bootstrap样式：

<nav class="navbar navbar-default sidebar" role="navigation">
    <div class="container-fluid">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-sidebar-navbar-collapse-1">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>      
        </div>
        <div class="collapse navbar-collapse" id="bs-sidebar-navbar-collapse-1">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Home<span style="font-size:16px;" class="pull-right hidden-xs showopacity glyphicon glyphicon-home"></span></a></li>          
                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Level 1<span class="caret"></span><span style="font-size:16px;" class="pull-right hidden-xs showopacity glyphicon glyphicon-chevron-down"></span></a>
                    <ul class="dropdown-menu forAnimate" role="menu">
                        <li><a href="#">Level 1.1</a></li>
                        <li class="dropdown-submenu">
                            <a href="#" class="dropdown-toggle" data-toggle="dropdown">Level 1.2<span class="caret"></span></a>
                            <ul class="dropdown-menu forAnimate" role="menu">
                                <li><a href="#">Level 1.2.1</a></li>
                                <li><a href="#">Level 1.2.2</a></li>
                                <li class="dropdown-submenu">
                                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Level 1.2.3<span class="caret"></span></a>
                                    <ul class="dropdown-menu forAnimate" role="menu">
                                        <li><a href="#">Level 1.2.3.1</a></li>
                                        <li><a href="#">Level 1.2.3.2</a></li>
                                        <li><a href="#">Level 1.2.3.3</a></li>
                                    </ul>
                                </li>
                            </ul>
                        </li>
                    </ul>
                </li>
            </ul>
        </div>
    </div>
</nav>

注意，需要引入Bootstrap的相关文件。同时，为了实现折叠效果，需要引入jQuery和Bootstrap的JavaScript文件。