JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="test"></div>
</div>

CSS

body, html {
  height: 100%;
}
.container {
  display: flex;
  justify-content: center; /* center horizontally */
  align-items: center; /* center vertically */
  height: 100%;
}
.test {
  background-color: black;
  width: 300px;
  height: 300px;
}