Question

It should just basically be an outline of the square or circle – that I can style accordingly (i.e. change the color to whatever I want, change the thickness of the border, etc.)

I would like to apply that circle or square over something else (like an image or something) and the middle part should be hollowed out, so you can see the image beneath the square or circle.

I would prefer for it to be mainly CSS + HTML.

Answer 1

Try This

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>

More here