JSFiddle - React, Tailwind, and code Playground

HTML

<div id="element"></div>

CSS

#element { 
    position: relative;
    width: 100px;
    height: 100px;
    background-color: blue;
    z-index:1; /* <-------- */
}

#element::after {
    content: "";
    width: 150px;
    height: 150px;
    background-color: red;

    /* create a new stacking context */
    position: absolute;
    z-index: -1;
}