JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

#a{
	opacity:0;
	display:none
}
#a.a{
	display:block;
	opacity:1;
}
#a,#a.a{
	transition:opacity ease 400ms;
}

JavaScript

setTimeout('a.classList.add("a");',1000);