Fallback is part of a support process when developing a website. Its objective is to provide an alternative to a technology we have applied to a website, but not all browsers support this specific feature.
This term may be split into polyfills and webshims.
A polyfill is a specific code that emulates a specific feature for browsers that do not support it natively. Polyfills always try to mimic the original browser feature, but there are a few cases where it may cause slight side effects such as an increase in the loading time or loss of performance.
An example of a polyfill is the html5shiv script that we simply drop in the code and it will act as if nothing changed. We will talk about html5shiv later.
Shims provide a fallback, but ...