JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
  <div id="middle">
    <div id="inner">
    x
    </div>
  </div>
</div>

CSS

#outer {
  display: flex;
  flex-direction: column;
  background-color: white;
  height: 300px;
}

#middle {
  flex-grow: 1;
  background-color: beige;
  height: 100%;
}

#inner {
  background-color: black;
  height: 100%;
}