JSFiddle - React, Tailwind, and code Playground

HTML

<div class="div1">
  <div class="div2">This div element has is 20% of the parent;
  </div>
</div>

CSS

div.div1 {
    /* max-width:500px; */
    border: 3px solid #000000;
}

div.div2 {
    width:20%;
    border: 3px solid #999999;
}