Mountaintop Corners
http://alistapart.com/articles/mountaintop/

Cederholm, Dan
List Apart, A
2004

Abstract:

Most of us have experience creating 'rounded' corners by erasing pixels. It’s a rudimentary web design technique — or so we always thought. But in the hands of Dan Cederholm, author of Web Standards Solutions, this seemingly simple technique paves the way for boxes and borders that can change sizes and colors at your whim.