<div class="parent">
 <div class="child">
  center me!
 </div>
</div>

/* Old way */
.parent {
 height: 100vh;
 width: 100vw;
 position: relative;
}

.child {
 position: absolute;
 left: 50%;
 top: 50%;
 transform: translate(-50%, -50%);
}