Overlap opacity

HTML

<div class="foo" style="position:absolute;left:10px;top:10px;width:100px;height:100px;">
  
</div>

<div class="foo" style="position:absolute;left:10px;top:10px;width:50px;height:200px;">
  
</div>

CSS

.foo {
  background-color: #555;
  opacity: 0.2;
}