JSFiddle - React, Tailwind, and code Playground

HTML

<div id="text">Hello World</div>

<div id="box">

    <div id="opaque"></div>
    
</div>

CSS

#box {
    height:200px;
    width:200px;
    border:5px solid black;
}

#opaque {
    position:absolute;
    width:200px;
    height:200px;
    background-color:black;
    opacity:0;
    -webkit-transition: opacity 0.3s ease-in-out;
	-moz-transition: opacity 0.3s ease-in-out;
	-ms-transition: opacity 0.3s ease-in-out;
	-o-transition: opacity 0.3s ease-in-out;
	transition: opacity 0.3s ease-in-out;
}

#opaque:hover {
    opacity:0.6;
}

#text {
    position:relative;
    color:blue;
    top:25px;
    left:10px;
    z-index:5;
}