刚刚看了bootstrap的导航栏，发现有点弄混了，现在来整理一下；
导航栏是一个很好的功能，是 Bootstrap 网站的一个突出特点。导航栏在您的应用或网站中作为导航页头的响应式基础组件。导航栏在移动设备的视图中是折叠的，随着可用视口宽度的增加，导航栏也会水平展开。在 Bootstrap 导航栏的核心中，导航栏包括了站点名称和基本的导航定义样式。
一、简单的ul,li组成的导航：
<ul class="nav nav-pills justify-content-center bg-dark nav-dark">
<li class="nav-item">
<a href="#" 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" 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" class="nav-link">1</a>
</li>
<li class="nav-item">
<a href="#" 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" 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" class="nav-link">2</a>
</li>
<li class="nav-item">
<a href="#" 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" 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" class="nav-link">3</a>
</li>
</ul>
//解析：.nav .nav-item .nav-link表示ul li是导航——.nav-pills表示胶囊状导航——.bg-dark/.nav-dark表示黑底白字——.justify-content-center表示导航栏在浏览器上居中显示
在浏览器上显示为：
缩小浏览器窗口显示为：
二、导航栏——<nav>标签中class="navbar navbar-expand-sm"——<ul>中class="navbar-nav"——<li>中class="nav-item"