Polymer (library)

Polymer is an open-source JavaScript library for building web applications using Web Components. The library is being developed by Google developers and contributors on GitHub. Modern design principles are implemented as a separate project using Google's Material Design design principles - wikipedia

Polymer is used by a number of Google services and websites, including the redesigned YouTube, YouTube Gaming, the redesigned Google Earth, Google I/O websites, Google Play Music, redesign of Google Sites and Allo for web.

Polymer is a lightweight library that helps you take full advantage of Web Components - polymer-project.org

With Web Components, you can create reusable custom elements that interoperate seamlessly with the browser’s built-in elements, or break your app up into right-sized components, making your code cleaner and less expensive to maintain - github

Polymer sprinkles a bit of sugar over the standard Web Components APIs, making it easier for you to get great results.

Polymer App Toolbox helps you build and deliver cutting-edge Progressive Web Apps with minimal overhead and payload, by leveraging powerful web platform features like Web Components, Service Worker and HTTP/2. The Toolbox provides a component-based architecture, responsive layouts, a modular router, localization support, turnkey support for local storage and offline caching, and efficient delivery of unbundled app resources. Adopt these features individually, or use them together to build a full-featured Progressive Web App.

The polymer cli tool for Polymer projects and Web Components - github