JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

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

// 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";