Re: [fetch] Mechanism to indicate "destination context" (#64)

@annevk I think there is an important difference between #48 and what we're trying to solve here. Specifically, for #48 we're talking about internal plumbing that allows us to explain `<img>` and friends via fetch.. as such, there is some internal API there that allows the `context` value to be set. 

By contrast, what we're talking about here is a developer-facing API, which can't allow the `context` to be overridden, because that would effectively allow anyone to bypass connect-src. Further, for cases like prefetch and prerender, the actual policy is not knowable on the current page.. to know it we have to wait to navigate to the destination and only then can we reason about what policies should be applied against the response.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/64#issuecomment-111247139