[рони]

exec,

<style type="text/css">
div {position: relative;box-sizing: border-box;}
div div {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;
}
</style>
<div style="width: 500px; height: 350px; border: 3px solid black;">
  <div id="aligned" style="width: 100px; height: 100px; background-color: #cccccc;">
  </div>
</div>