It is impossible. The features required to make browser Javascript safe for crypto aren't even on the roadmaps of browser vendors.
Your reason for using browser Javascript for crypto --- here, Recurity's JS PGP implementation --- is the same as every other JS crypto project's reason: doing everything in the browser makes it easier for users to adopt your project. You are not the first person to point this out and you won't be the last.
The problem is, you acknowledge one side of this design ("it's super easy for users") and ignore the other ("it's fatal to security").
Here, you even acknowledge that there was a simple mechanism available to you that might have marginally improved security (packaging the whole application as a Chrome extension). But that made your life too hard! So you abandoned that idea and just made all the crypto downloadable from the server on HTML pages!
Your reason for using browser Javascript for crypto --- here, Recurity's JS PGP implementation --- is the same as every other JS crypto project's reason: doing everything in the browser makes it easier for users to adopt your project. You are not the first person to point this out and you won't be the last.
The problem is, you acknowledge one side of this design ("it's super easy for users") and ignore the other ("it's fatal to security").
Here, you even acknowledge that there was a simple mechanism available to you that might have marginally improved security (packaging the whole application as a Chrome extension). But that made your life too hard! So you abandoned that idea and just made all the crypto downloadable from the server on HTML pages!