JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a"> </div>

<div class="b">
  <h1>Hello</h1>
</div>

CSS

.a {
  background: pink;
  padding: 10px;
}

.b {
  background: gray;
}

JavaScript

setTimeout(function() {

document.querySelector('.a').appendChild(document.querySelector('.b'))
}, 1000)