A polyfill is like spackle for your Web sites, it’s a way to determine if a given HTML5 feature is available to the user currently browsing your site, and to provide either a shim that ‘fills in’ that support or a course of graceful degradation that enables your site to still function fully. A polyfill can be nearly anything – a JavaScript library that adds support for CSS3 selectors to old versions Internet Explorer. So here are some HTML5 and CSS3 polyfill solutions for your site to provide fallback functionality to older browser.

HTML5 Cross Browser Polyfills

Great collection of all the shims, fallbacks, and polyfills in order to implant html5 functionality in browsers that don’t natively support them.

yepnopejs

Yepnope is an asynchronous conditional resource loader that’s super-fast, and allows you to load only the scripts that your users need.

Grid Style Sheets

GSS reimagines CSS layout & replaces the browser’s layout engine with one that harnesses the Cassowary Constraint Solver.

Myth

Myth is a preprocessor that lets you write pure CSS without having to worry about slow browser support, or even slow spec approval. It’s like a CSS polyfill.

CSSRegions.js

CSSRegions.js is an experimental polyfill that uses JavaScript to bring CSS Regions functionality to browsers that don’t support the feature.

Polyfill.js

Polyfill.js is a library designed to make writing CSS polyfills much, much easier. It’s an abstraction library that takes care of the boilerplate, so you can focus on what your polyfill actually does.

HandJS

Hand.js is a polyfill that will allow you to write your code about touch only once even if the browser does not support pointer events.

Picturefill

Smashingmag wrote very nice article about perfect polyfill solution for responsive images that you can find here.