JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
  width: 300px;
  height: 300px;
  transition: background-color 3s;
  background-color: rgb(0, 0, 255);
}

div.change-color {
  background-color: rgb(255, 0, 0);
}

JavaScript

setTimeout(function() {
	document.getElementsByTagName('div')[0].classList = 'change-color';
}, 3000)