SOURCE

<div class="parent">
  <div class="child">123</div>
</div>

.parent{
  width:200px;
  height:100px;
  background:gray;
}
.child{
  width:50%;
  height:0px;
  background:#ffffcc;
  color:green;
  /*padding-right:10px;
  padding-top:10px;
  padding-left:10px;
  padding:2px;*/
  padding-top:50%;
}