Re: [ServiceWorker] Interception of the fallback requests initiated from a SW for cross-origin resources (#684)

> Registrations overwrite each other and last-one-wins semantics prevail.

Furthermore, if a new version fails to call `handleFallThroughRequests` (or whatever we call it), then another SW won't be used for fall-through requests.

> should allow b.com's onfallthroughfetch to see CORS preflights

I still don't think this is necessary. The fall-through worker will be able to inspect the request and should be given details on the origin that will get access to the response. It'll be able to make a call without preflight access.

> responses from onfallthroughfetch should probably be made opaque by default 

I'm not convinced opaque responses from a third party services are useful. One of the key usecases we have is fonts, and they require CORS. Isn't the new event name enough of an explicit action?

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