JSFiddle - React, Tailwind, and code Playground

HTML

<h1>I'm an h1</h1>
<h2>I'm an h2</h2>
<p>I'm a p</p>
<section>
  <h1>I'm an h1 in a section</h1>
  <h2>I'm an h2 in a section</h2>
  <p>I'm a p in a section</p>
</section>

JavaScript

window.setTimeout(function showFontSize(){
  var els = document.querySelectorAll('h1, h2, p');
  for (var i=0; i<els.length; i++){
		els[i].innerHTML += ' <span style="font-size:16px">font-size=' + window.getComputedStyle(els[i],null).getPropertyValue("font-size") + "</span>";
  }}
, 100);