JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a"></div>

CSS

#a {
	background-color: rgba(0,0,0,0.9);
	width: 100px;
	height: 100px;
	transition: all 3s ease 3s;
}
#a.fade {
	background-color: rgba(0,0,0,0);
}

JavaScript

$(function(){
    $('#a').addClass('fade');
});