JSFiddle - React, Tailwind, and code Playground

HTML

<div>Test</div>

CSS

div {
    width: 50px;
    height: 50px;
    background: black;
    position: relative;
}

div:after {
    content: '';
    background: red;
    display: block;
    width: 30px;
    height: 30px;
    position: absolute;
    top: 5px;
    right: 5px;
    opacity: 0;
    -moz-transition: opacity 0.4s linear;
    -webkit-transition: opacity 0.4s linear;
    transition: opacity 0.4s linear;
}

div:hover:after {
    opacity: 1;
}