What are some alternatives?
When comparing CanJS and Polyfill, you can also consider the following products
Vue.js - Reactive Components for Modern Web Interfaces
Polymer - Polymer is a library that uses the latest web technologies to let you create custom HTML elements.
Aurelia - Next gen JS framework written with ES6 and ES7. Integrates with Web Components. No external dependencies except polyfills.
Sails.js - Realtime MVC Framework for Node.js