Jeff mentioned the concept of \'Progressive Enhancement\' when talking about using JQuery to write stackoverflow.
After a quick Google, I found a couple of high-leve
Going at it from the other direction is sometimes referred to as graceful degradation. This is usually needed when the site is built first with the enhanced functionality afforded by the various technologies then modified to degrade gracefully for browsers with those technologies are not available.
It is also graceful degradation when designing to work with older browsers (ancient in the Internets terminology) such as IE 5.5, Netscape, etc...
In my opinion it is much more work to gracefully degrade the application. Progressively enhancing it tends to be much more efficient; however, sometimes the need to take an existing app and make it accessible in these lacking environments arise.