JSFiddle - React, Tailwind, and code Playground

HTML

<style>
 h3:after {
   content: ' rocks!';
 }
</style>

<h3>generated content</h3> 

<script>
  var h3       = document.querySelector('h3'), 
      result   = getComputedStyle(h3, ':after').content;

  alert('the generated content is: ', result); // returns ' rocks!'
</script>