<div class="layer">
  <span class="content">Layer Contents</span>
</div>
body{background:#ccc}
.layer{
  position:absolute;
  top:0;right:0;bottom:0;left:0;
  display:flex;
  align-items:center;
  justify-content:center;
  
  display:-webkit-flex;
  -webkit-align-items:center;
  -webkit-justify-content:center;
}
.layer .content{background:#f00}
Rerun