<header id="header">
  <nav id="nav" class="nav">
    <a href="index.html">Home</a>
  </nav>
</header>
html, body {
  display: grid;
  align-content: center;
  
  height: 100%;
}

.nav {
  display: flex;
  padding: 25px;
}

.nav > a {
  padding: 0 15px;
}
// Create a new anchor element
const $about = document.createElement('a')
$about.href = 'about.html'
$about.textContent = 'About'

// Append the new anchor to the Nav
const $nav = document.getElementById('nav')
$nav.appendChild($about)

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.