JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
	<div class="ok"></div>	
</div>

CSS

.main {
	width: 200px;
	height: 200px;
	background: black;
}
.ok {
	width: 50px;
	height: 50px;
	background: red;	
	transition: opacity 1s linear;	
    opacity: 0;
}
.main:hover .ok{ 
	opacity: 1;
}