SOURCE

<div class='div-wrap'>123123</div>

html,body{
  margin:0;
  padding:0;
  height:100%;
  width:100%;
}
body{
  /* display:flex; */
  /* align-items:center; */
  /* justify-content:center; */
}
.div-wrap{
  background-color:red;
  width: 200px;
  height: 200px;
  position: absolute;
  top: 0;left: 0;right: 0;bottom: 0;
  margin: auto;
}