JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
  <div id="popup">
    yolo
  </div>
 </div>

CSS

#container {
  height: 100vh;
  padding: 0;
  margin: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px solid grey;
}

#popup {
  width: 250px;
  height: 250px;
  border: 1px solid red;
  text-align: center;
}