JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

html, body {
  margin: 0;
  height: 100%;
}

#child {
  width: 200px;
  height: 200px;
  background: black;
}

#child {
  position: absolute; /* fixed */
  top: 0;
  bottom: 0;
  margin: auto 0;
}