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

.container{
  position:relative;
  width:90vw;
  height: 90vh;
  border: 3px solid steelblue;
  margin: auto;
}

.element{
  position:absolute;
  top: 0; bottom: 0; left: 0; right: 0;
  margin: auto;
  background: tomato;
  width: 50vw; height: 50vh;
}

/* just aesthetics */
body{
  height: 100vh;
  display:flex;
  background: azure;
}