JSFiddle - React, Tailwind, and code Playground

HTML

<div> 
    <div>
    </div>
</div>

CSS

div {
    width: 200px;
    height: 200px;
    background: olive;
}

div > div {
    width: 100px;
    height: calc(100% - 10px);
    background: azure;
}