Whether it's a good addition to the language or not is debatable, but as a browser feature, it sounds terrifying. Browsers have a huge attack surface as it is (I mean, WebGL is a thing – exposing GPU drivers to random untrusted code on the internet, what a brilliant idea...), and exposing threading will only make it worse. Every single API would have to be carefully audited and made thread-safe, and I'm sure that many 'fun' bugs would crop up as a result.