为何打完下面这个代码，网页是这样的？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>全局样式</title>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
</head>
<body>
<!-- Main jumbotron for a primary marketing message or call to action -->
<div class="jumbotron">
<h1>Hello, world!</h1>
<p>This is a template for a simple marketing or informational website. It includes a large callout called a jumbotron and three supporting pieces of content. Use it as a starting point to create something more unique.</p>
<p><a href="#" >Learn more »</a></p>
</div>
</body>
</html>
