Div Overlap

HTML

<div class='a'>back div</div>
<div class='b'>back div asdf a</div>
<div class='a'>This div is on the top</div>

CSS

.a {
  width: 50px;
  border: 1px solid red;
}

.b {
  width: 50px;
  border: 5px solid green;
}