CodePen Embed - position: absolute; + transform;

<div class="parent">
  <div class="child"></div>
</div>

.parent {
  position: relative;
  background: lightyellow;
  height: 100vh;
}
.child {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translateY(-50%) translateX(-50%);
  width: 150px;
  height: 80px;
  background: lightskyblue;
}

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