JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div {
    width: 80%;
    height: 300px;
    background-color: red;
}

div::after {
    content:'';
    position: fixed;
    height: 300px;
    width: 40%;
    left: 40%;
    background: blue;
}