固定导航栏
导航栏可以固定在头部或者底部。我们使用 .fixed-top 类来实现导航栏的顶部固定，
实例：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap4 实例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" rel="external nofollow" target="_blank" >
<script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" ></script>
<script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" ></script>
<script src="https://cdn.bootcss.com/bootstrap/4.1.0/js/bootstrap.min.js" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" ></script>
</head>
<body style="height:1500px">
<nav class="navbar navbar-expand-sm bg-dark navbar-dark fixed-top">
<a class="navbar-brand" href="#">Logo</a>
<ul class="navbar-nav">
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
</ul>
</nav>
<div class="container-fluid" style="margin-top:80px">
<h3>固定导航栏</h3>
<p>导航栏可以固定在头部或者底部。</p>
<h1>滚动页面查看效果。</h1>
</div>
</body>
</html>
.fixed-bottom 类用于设置导航栏固定在底部：
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap 实例</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/css/bootstrap.min.css" rel="external nofollow" target="_blank" >
<script src="https://cdn.staticfile.org/jquery/3.2.1/jquery.min.js" rel="external nofollow" ></script>
<script src="https://cdn.staticfile.org/popper.js/1.15.0/umd/popper.min.js" rel="external nofollow" ></script>
<script src="https://cdn.staticfile.org/twitter-bootstrap/4.3.1/js/bootstrap.min.js" rel="external nofollow" ></script>
</head>
<body style="height:1500px">
<nav class="navbar navbar-expand-sm bg-dark navbar-dark fixed-bottom">
<a class="navbar-brand" href="#">Logo</a>
<ul class="navbar-nav">
<li class="nav-item">
<a class="nav-link" href="javascript:void(0)">Link</a>
</li>
<li class="nav-item">
<a class="nav-link" href="javascript:void(0)">Link</a>
</li>
</ul>
</nav>
<div class="container-fluid">
<h3>底部固定导航栏</h3>
<p>导航栏可以固定在头部或者底部。</p>
<h1>滚动页面查看效果。</h1>
</div>
</body>
</html>
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