Since web browsers have been around as long as the Web, it is possible to have customers viewing your web pages in browsers that are extremely old and missing features of more modern browsers. Graceful degradation is a strategy of handling web page design for different browsers.
A web design that is built to gracefully degrade is intended to be viewed first by the most modern browsers, and then as older, less feature-rich browsers view it it should degrade in a way that is still functional, but with fewer features.
The main difference between the graceful degradation strategy and progressive enhancement is where you start your design. If you start with the lowest common denominator and then add features for more modern browsers for your web pages, you're using progressive enhancement. If you start with the most modern, cutting edge features, and then scale back, you're using graceful degradation.
Graceful Degradation Doesn't Mean Telling Your Readers, "Download the Most Recent Browser"
One of the reasons many modern designers don't like the graceful degradation approach is because it often turns into a demand that readers download the most modern browser for the page to work. But this is not graceful degradation. If you find yourself wanting to write "download browser X to get this feature to work", you have left the realm of graceful degradation and moved into browser-centric design.
A good rule of thumb is to follow the same rules for graceful degradation as you would for progressive enhancement:
- Write valid, standards-compliant HTML
- Use external style sheets for your designs and layout
- Use externally linked scripts for interactivity
Then go out and build the most cutting-edge design you can! Just make sure that it degrades in less functional browsers but still works.