JSFiddle - React, Tailwind, and code Playground

HTML

<flex-container>
  <flex-item></flex-item>
</flex-container>

CSS

flex-container {
  display: flex;
  /* align-items: center; */
  width: 100%;
  height: 600px;
  background-color: gray;
  position: relative;
}

flex-item {
  background-color: white;
  width: 100px;
  height: 100px;
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
}