Polymer 1.7.0 provides forward compatibility features that let you build elements that run under both 1.0 and 2.0.

The primary goal of the Polymer 2.0 release is to take advantage of native, cross-browser support for Web Components.

Polymer 1.x is built on top of the so-called "v0" Web Components specs, which are supported natively only in Google Chrome; using Polymer in other browsers has always required the use of polyfills.

With Custom Elements v1, web developers can create new HTML tags, beef-up existing HTML tags, or extend the components other developers have authored. The API is the foundation of web components. It brings a web standards-based way to create reusable components using nothing more than vanilla JS/HTML/CSS. The result is less code, modular code, and more reuse in our apps.

Most of the bigger web project are designed to be modular. We all know that Custom Elements spec provides some modularity on individual HTML Element level, but is that all Web Components can give?

Using JavaScript with ShadowDOM, templates, and the other specs that together make up the web components ecosystem, you turn that inert span-like element into something all-singing and dancing. That’s great if the browser supports those technologies, and the JavaScript executes successfully. But if either of those conditions aren’t met, what you’re left with is basically a span.

Adam Onishi's thoughts on how Web Components will change the way we build websites and how Progressive Enhancement should be handled by them.