JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">
    some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here some stuff in here 
</div>

CSS

#a{position:relative}
#a:after{
    content:" ";
    position:absolute;
    display:block;
    top:0;left:0;right:0;bottom:0;
    background-color:#f00;
    z-index:-1;
    opacity:0.2;
}