JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
    <div></div>
</div>

CSS

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