JSFiddle - React, Tailwind, and code Playground

HTML

<h1>H1</h1>
<h1>H2</h1>
<p class="large">large paragraph</p>

CSS

body {
  font-size: 1em;
}

h1 {
  font-size: 1.5em;
}

h2 {
  font-size: 1.3em;
}

p.large {
  font-size: 1.2em;
}

body.enlarged {
  font-size: 1.3em;
}

JavaScript

document.body.onclick = function() {
  document.body.classList.add('enlarged'); 
}