JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<div class="modal-box">
this is the modal
</div>
</body>

CSS

.modal-box {
  position: fixed;
  top: 50%;
  left: 50%;
  transform:translate(-50%,-50%);
  background: #aaa;
}

body {
  height: 500px;
  width: 500px;
  background: #eee;
}