CSS Design: Custom Underlines
http://alistapart.com/articles/customunderlines/

Robertson, Stuart
List Apart, A
2004

Abstract:

While web designers generally have a great deal of control over how a document should be presented, basic CSS doesn't provide many options for the style of underlines below the links on a page. But with a few nips and tucks, you can take back creative control of the way your links look.