JSFiddle - React, Tailwind, and code Playground

HTML

<h3 foo="bar">generated content</h3>

CSS

h3:after {
   content: ' rocks!' attr(foo);
 }

JavaScript

var h3       = document.querySelector('h3'), 
      result   = getComputedStyle(h3, ':after').content;
 
  console.log('the generated content is: ', result); // returns ' rocks!'