JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

html,body { height: 100%; }
#parent { position: relative; width: 50%; height: 50%; border: 1px solid black; }
#child { position: absolute; top: 0; bottom: 0; left: 0; right: 0; background: cyan; }