JSFiddle - React, Tailwind, and code Playground

HTML

<p id="start">...</p>
<p id="finish">...</p>

JavaScript

var start = document.getElementById("start");
start.innerHTML = "Started";

setTimeout(function() {
  // This will block the browser
  var sum = 0;
  var reps = 50000;
  for (i = 0; i < reps; i++) {
    for (j = 0; j < reps; j++) {
      sum += i + j;
    }
  }
  var finish = document.getElementById("finish");
	finish.innerHTML = "Finished";
}, 100);