JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    a
    </div>

CSS

div {
        height:300px;
        color:red;
        position:relative;
    }
    
    div:before {
        content:'';
        display:block;
        box-shadow:inset 0 0 10px black;
        position:absolute;
        top:0;
        left:0;
        width:100%;
        height:100%;
    }