JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myPage">
    <p>This will be covered by the overlay</p>
    <div id="myOverlay"></div>
</div>

CSS

#myPage {
    position: relative;
    height: 300px;
    width: 200px;
}

#myOverlay {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    z-index: 1000;
    background: #333;
    opacity: .3;
}