I love it. It'll be interesting to see if browser vendors will start attempting to detect these sorts of polyfill services and respond with their own browser-bundled or -specific polyfills.
Yyyyeah uh, a browser-implemented polyfill is also called a 'supported JS feature'. Polyfills are for older browsers that don't support a certain feature yet (or I guess for features that are not yet implemented in newer browsers).
If browser vendors adopted this this opt-in, polyfill-first approach it would make it significantly less expensive and much simpler to progressively rollout experimental features to the public.