<div class="d-flex justify-content-center align-items-center">
<div class="h2">Header 2</div>
<div>
html, body {
  display: grid;
  height: 100%;
}

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.