Re: [w3c/manifest] Add a unique identifier for a PWA (#586)

> This does bring me back to an earlier point though - if id is a field inside the manifest what happens if two different web app manifests claim the same id? Is it not possible for user agents to get stuck in an update loop where they keep switching between two or more manifests linked from different web pages as the user happens to navigate to them? This is arguably another benefit of making the manifest URL the global identifier because that can't easily happen.

That's the reason we force the id to prefix with the start_url's origin. So if within the same origin, they serve multiple manifests with the same id , yeah they will be recognized as the same app. But I think that should be expected by the app developers for that site as this is spelled out in the spec.

This does bring more work for PWA stores that do web scrawls without curation - they need to verify that the document of the start_url links to the manifest_url, if the manifest is at a different origin. I think this is the right thing to do though. Also I don't know which PWA store scrawl the web without any validation process right now.

> Also, I see the id as becoming recommended, even to the point of us eventually requiring it for installability. Once everyone has an id set, then the world becomes a whole lot simpler. These defaults basically just help existing apps update to the new id world.

+1. I think with `id` added to manifest, this should be the recommended way. And the default is just a fallback mechanism that's optimized for backward compatibility. With `scope` it doesn't really give us backward compatibility benefit. With manifest_url, for cross-origin and data url cases, also doesn't allow backward compatibility.


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