JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

html, body {width:100%;height:100%;}
.parent {
    display: flex;
    width:100%;
    height: 100%;
    border: 5px solid red;
}

.child {
  width: 100px;
  height: 100px;
  margin: auto;
  border: 5px solid blue;
}