Add custom features to browsers old and new by writing polyfill libraries, JavaScript plugins that take browsers beyond their native capabilities. In this practical fieldbook, author Brandon Satrom introduces principles and guidelines for polyfill development, and then walks you through the steps for building a complex, real-world HTML5 polyfill. You'll also explore the future of polyfilling - or prollyfilling - that will enable you to test and work with emerging concepts, often ahead of browser vendors. By the time you finish this book, you'll have the tools and hands-on experience you need to build reliable polyfills for today's and tomorrow's Web. Learn the current state of polyfills, including shims, opt-ins, and drop-ins Use principles and practices to build responsible polyfills that benefit the entire web development community Build out several features for an HTML5 Forms polyfill library Configure a build environment and run automated cross-browser testing Optimize performance, handle edge cases, and fine-tune the speed of your polyfill Get examples of prollyfilling libraries that push the boundaries of the Web Write a sample prollyfill and compare it to current polyfill builds