Edit in JSFiddle

<div class="parent">
  <div class="child">Hello World!</div>
</div>

.parent {
  display: table;
  height: 100px;
  width: 100%;
  text-align: center;
  border: 2px dashed #f69c55;
}
.child {
  display: table-cell;
  vertical-align: middle;
}