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<!DOCTYPE html>
<html lang="en">
 <head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<link rel="stylesheet" href="./bootstrap/css/bootstrap.min.css">
<script src="./jquery/jquery-3.4.1.min.js"></script><!--引入js文件-->
<script src="./bootstrap/js/bootstrap.js"></script>
 </head>
<body>
<nav class="navbar navbar-default" role="navigation">
    <!-- Brand and toggle get grouped for better mobile display -->
    <div class="navbar-header">
        <!--汉堡-->
        <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-ex1-collapse">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
        </button>
        <!--标题-->
        <a class="navbar-brand" href="#">Title</a>
    </div>   
    <!-- Collect the nav links, forms, and other content for toggling -->
    <!--汉堡展开-->
    <div class="collapse navbar-collapse navbar-ex1-collapse">
        <ul class="nav navbar-nav">
            <li class="active"><a href="#">Link</a></li>
            <li><a href="#">Link</a></li>
        </ul>
        <form class="navbar-form navbar-left" role="search">
            <div class="form-group">
                <input type="text" class="form-control" placeholder="Search">
            </div>
            <button type="submit" class="btn btn-default">Submit</button>
        </form>
        <ul class="nav navbar-nav navbar-right">
            <li><a href="#">Link</a></li>
            <li class="dropdown">
                <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown <b class="caret"></b></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><a href="#">Separated link</a></li>
                </ul>
            </li>
        </ul>
    </div>
</nav> 
</body>
</html>

nav:要用role="navigation"
data-toggle="collapse":属性点击可以展开，隐藏。
data-target=".navbar-ex1-collapse":指向。下面有div的class属性。
collapse：隐藏，折叠。
.navbar-inverse:类可以改变导航条的外观。
body { padding-top: 70px; }:固定的导航条会遮住页面上的其它内容，除非你给 < body>元素底部设置了 ,padding。用你自己的值，或用下面给出的代码都可以.\

导航

导航

结果