Yeah, but I don't think any of these mime-types represent any threat for today's Javascript. OTOH Dart(or Dash)'s sponsor already gave us some clues of that their intentions are. I quote https://gist.github.com/1208618 "Push for Dash to become an open standard and be adopted by other browsers. Developers using Dash tooling will be able to use a cross-compiler to target Javascript for browsers that do not support Dash natively." again, it says "target Javascript for browsers that do not support Dash natively [for now]."