JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div{
    position: absolute;
    top: 10px;
    left: 10px;
    height: 20px;
    width: 20px;
    background: red;
    z-index: 10;
}
div:before{
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    height: 40px;
    width: 40px;
    background: blue;
    z-index: 1;
}