A polyfill is a term coined by Remy Sharp for a snippet of code that adds support for a feature to browsers that don’t offer it natively.
They let you write code once and provide deeper backwards compatibility and browser support—without having to use a clunky preprocessor or command line tool.
How to use a polyfill
Alternatively, you can automatically polyfill your site with a service like polyfill.io.
Some were written by me. Others were squirreled away from around the web. Links to the source and attribution are included with each.