JSFiddle - React, Tailwind, and code Playground

HTML

<title>AB to EF</title>
    <script src="popup.js"></script>
  <body>
    <h1>AB to EF Converter</h1>
    <button id="turnOn">Turn On</button>
    <button id="turnOff">Turn Off</button>
  </body>

JavaScript

var markup = document.documentElement.innerHTML;

document.addEventListener('DOMContentLoaded', function() {
  var checkPageButton = document.getElementById('turnOn');
  checkPageButton.addEventListener('clickOn', function() {

		

    chrome.tabs.getSelected(null, function(tab) {
      markup.replace(/ab/, "ef")
    });
  }, false);
}, false);