How do I create a circle or square with just CSS – with a hollow center?

div.circle {
  -moz-border-radius: 50px/50px;
  -webkit-border-radius: 50px 50px;
  border-radius: 50px/50px;
  border: solid 21px #f00;
  width: 50px;
  height: 50px;
}

div.square {
  border: solid 21px #f0f;
  width: 50px;
  height: 50px;
}

<div class="circle">
  <img/>
</div>
 <hr/>
<div class="square">
  <img/>
</div>