It doesn't (though one might argue that as a result we will see much fewer JS implementations of AES coming out). What I am talking about is a better way to distribute the code, the ability to verify the code, the ability to sandbox it, and a proper secure crypto implementation to rest this on.

Basically, I want the contents of GMail messages (ones I compose and ones I read) to be rendered in a sandbox to which the rest of the page has no access. Moreover, I want to get the code that makes the sandbox tick to not come from Google, but from a third part I actually trust.

Once again, this starts looking a lot like normal downloaded apps with package signing, and less like the web. I am not certain there is a middle ground between the two that provides both security and convenience.