<div class="o">
  <div class="i"></div>
</div>
.i {
  width: 50px;
  height: 50px;
  margin: auto;
  background: red;
}

.o {
  display: flex;
  height: 100vh;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.