<div class="container">
  <div class="element">Centered content</div>
</div>

.container{
  display: table;
}
.element{
  display: table-cell;
  text-align: center;
  vertical-align: middle;
  background: tomato;
  height: 200px; width:200px;
  color: white; font-size: 2rem;;
}
/* just aesthetics */
.container{
  width:90vw;
  height: 90vh;
  border: 3px solid steelblue;
  margin: auto;
}

body{
  height: 100vh;
  display:flex;
  background: azure;
}