Re: [w3c/manifest] Feature request: "isolate web app" hint (Issue #1109)

None of this is supposed to be binding (it's just a hint), so we shouldn't get into a situation where a site can depend on `"no-isolate"` to guarantee that there won't be isolation.

Of course we need to be worried about de facto standards, e.g. what if a giant browser implements Policy 4 above, and it becomes de facto that `"no-isolate"` is respected, and a website using `"no-isolate"` makes an assumption that it won't be isolated?

Well we can't be concerned about that because today, all browsers except Safari implement Policy 1, which means all sites today can (and do) assume no isolation. So introducing `"no-isolate"` doesn't give sites a new assumption that they can't already make today.

> what's the value that users get from "no-isolate" that would motivate a user agent to support that? ... That's going to be the case regardless on MacOS/Safari, right?

The value would be that sites have a way to opt out of isolation for at least some browsers (but not Safari, which implements Policy 2 today). I see your point though: today we have no isolation option, we just have Safari implementing Policy 2 which breaks some sites. Today, there's impetus for sites to fix themselves to work in Policy 2 browsers. If we add `"no-isolate"` then sites may simply use that (fixing themselves for Policy 4 browsers) instead of properly being compatible with isolation.

The other side of this is that Policy 2 and 4 essentially violate the body of web standards as written (you can view the installed context as a "separate browser", but that "other browser" is broken in that it forwards all URLs outside of the app scope to the main browser, which breaks sites existing valid expectations based on what's written in the standards). Policy 4 would at least give sites a way to opt out of that broken behaviour.

I think the best course of action here is to not specify `"no-isolate"` (since it won't have any meaningful difference to `"auto"` in all known browsers), but choose a syntax that allows us to add it in the future if desired.

> So I wonder if we should address that problem instead, which would solve that for everyone?

I assume you're referring to address that by giving sites tools like being able to specify which URLs to handle within the app context, etc. I'm in favour of that. It may be the case that we don't need the opt-out if we can give sites tools to work properly when isolated.

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