SOURCE

<div class="outer">
  <div class="inner">
  </div>
</div>

.outer {
  height: 100px;
  font-size: 0;
  text-align: center;
  border: 1px dashed black;
}

.outer:before {
  content: '.';
  display: inline-block;
  height: 100%;
  vertical-align: middle;
}

.inner {
  display: inline-block;
  width: 50px;
  height:50px;
  vertical-align: middle;
  background-color: black;
}