Of course pure source transformations (classes, let, const, splats and such) just yield ES5 source directly, and so you lose native browser support for those. T

Of course pure source transformations (classes, let, const, splats and such) just yield ES5 source directly, and so you lose native browser support for those. That's no big deal since most browsers really aren't really up to scratch yet in ES6 support. I'm not sure about the runtime, but I think I remember it doing some (very basic) feature detection while browsing through the generated code during a debugging session. EDIT: Frankly, I think it would probably be a good idea to polyfill all browsers until they all really implement the ES6 spec.