Graceful Degradation
This is a design practice that is built around developing a website or application that will run in the latest browsers but also delivers requisite functionality and content in the older browsers. In other words, feature-rich websites are designed for new browsers that are also compatible with older ones. However, the user experience it delivers in the old browsers will not be on par with what is seen in the newest ones. You could say that the primary aim of graceful degradation is to ward off disastrous failure.
While polyfills are acceptable for building in missing features along with JavaScript, it is a good practice to provide other alternatives to styling, layout, and other features wherever it can be provided. HTML fallback behaviour or the CSS cascade can be used.
Graceful degradation is a valuable method that web developers can employ to create the most efficient websites, seeing how they are accessed by multiple unknown users on different kinds of devices, platforms, and browsers. Therefore, this philosophy is aligned with the basic ‘accessibility’ philosophy of the web; the idea being to develop websites that can be accessed by as many people as possible.
Graceful degradation becomes necessary because every technology does not improve simultaneously. Most browsers aren’t capable of handling today’s programming languages; all browsers are not able to support the same features, and so on.