刚开始的代码是这样的，使用react router4.x写的demo路由跳转后，页面的没有渲染，是因为没有引入外部css文件（或者说引入外部文件路径错误）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<meta name="theme-color" content="#000000">
<link rel="manifest" href="%PUBLIC_URL%/manifest.json">
<title>React App</title> <link rel="stylesheet" href="./css/bootstrap.css">
</head>
<body>
<noscript>
You need to enable JavaScript to run this app.
</noscript>
<div id="root"></div>
</body>
</html>
将上面代码（红色部分）的点去掉就可以了，如下所示
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<meta name="theme-color" content="#000000">
<link rel="manifest" href="%PUBLIC_URL%/manifest.json">
<title>React App</title> <link rel="stylesheet" href="/css/bootstrap.css">
</head>
<body>
<noscript>
You need to enable JavaScript to run this app.
</noscript>
<div id="root"></div>
</body>
</html>