Actually coffee's js is currently compatible down to IE6, which the author said is what he wants to support at this point. So unless MS were planning to backport ES6 compatible JavaScript to the dead IE6, CoffeeScript supporting ES6 will either change/break at some point or become a unreasonable backward-compatibility clusterfuck (probably the former). ES6 becomes official in late 2014 so there is probably enough time for a lot of changes to happen.