By default HTML element border is placed outside of element content. This is described in CSS box model:
But I've stumbled with case where new additional space outside element was undesirable.
So border must be placed inside element's content.
box-shadow property can set shadow inside of element. So you can simulate border using shadow:
border: 5px solid red;
box-shadow: 0 0 0 5px blue inset;
Gray - element content, red - standard border, blue - inner border (shadow).
So element space is taken by gray area plus blue shadow border. And red border takes additional (undesirable) space outside.
You can use CSS3
box-shadow to get effect of inner border. But I believe that another techniques exist to get the same result.