JSFiddle - React, Tailwind, and code Playground

HTML

<div id='cont'>
    <div id='opacity'></div>
    <div id='body'>wefwefwefwefwefwefe</div>
</div>

CSS

#body, #opacity  {
    width: 60%;
    height: 400px;
    z-index:2;
    position: relative;
}
#opacity {
    position: absolute;
    background: black;
    opacity: 0.5;
    z-index:1;
}