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

> They would have to use their old manifest url as their ID, forever. Or whatever we have as default. I guess we could have some custom spec language here around "if you didn't have an ID and you set one, then that is the ID, as long as the manifest url matches"?, but that might be complicated. Open to thinking about that though.

Yes. The point of having a hard-specified default is that site authors know exactly what their site used to default to, so if they want to change the thing that the default is based off (e.g., the manifest URL), they can set the ID to exactly the string that used to be the default, to avoid their ID changing.

We can put a non-normative note about this, but we don't need any normative text around the temporal changes to a manifest file.

> Also, it seems potentially weird to have multiple manifests all be part of the same app.

Well. "Multiple manifests" is a bit hard to define (if a manifest changes its content or its URL, is that "multiple manifests"?). Essentially the entire point of this ID _is_ to formally identify when a manifest change represents a new app, versus a mutation of an existing app.

Using a different manifest URL _is_ the [currently recommended and only viable way](https://www.w3.org/TR/appmanifest/#internationalization) to provide localized manifest metadata. So we have to support that, unless we want to block on #676 (properly supporting localization). I think this works fine: you would make all of your different-locale manifests have the same ID, so they all represent the same app. Whichever manifest was served at install time determines what language you see. That way, if the user changes their language, and the start URL starts pointing at another language's manifest URL, the browser's updater will go "aha, I'll update to a new version of the manifest" as opposed to "that app is not installed". (This is _exactly_ the point of having an ID, so we can distinguish those cases.)

Going back to what @benfrancis said, the same answer applies:
> What happens if two web app manifests provide the same ID?

The whole point of the ID is so that we know when "two web app manifests" represent two different apps versus two different versions of the same app.

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