JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container" class="container"></div>

CSS

.container {
height: 200px;
width: 200px;
background-color: salmon;
display: block;
border-radius: 5px;
opacity: 0;
transition: opacity 2s ease-in-out;
}

JavaScript

var container = document.querySelector("#container");
container.style.opacity = 1;