<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>CodePen</title>
</head>
<body>
  <button class="btn btn-primary">Button</button>
</body>
</html>

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css

External JavaScript

This Pen doesn't use any external JavaScript resources.