JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

#parent{    
    width:400px;
    height:400px;
    position:relative;    
    background: rgba(64, 64, 64, 0.5)
}

#child{
    position:absolute;
    opacity:1;
    width:200px;
    height:200px;
    background:blue;
}