Require document to be in scope to be styled by the manifest

[mgiuca]

This change (choose one):

• Breaks existing normative behavior (please add label "breaking")
• Adds new normative requirements
• Adds new normative recommendations or optional items
• Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
• Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

• Safari (link to issue)
• Chrome (link to issue)
• Firefox (link to issue)
• Edge (public signal)

Commit message:

Require document to be in scope to be styled by the manifest.

This introduces a small new normative requirement that the manifest NOT be
applied to a document that it outside of its scope. A note explains that this
only applies to the authority of the metadata, not the installation.

Adds a non-normative note that the app can be installed from a document that is
outside the scope of that app.

Closes #784.

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Preview | Diff

[mgiuca]

@dominickng @marcoscaceres Could you look over this language before circulating this more widely.

I think we should seek approval from several vendors on both of the changes included here (both the normative change to ban a manifest from applying to an out-of-scope document, and the explicit decision not to make normative changes to ban manifests from being installable from an out-of-scope document).

Note that this PR is based on #790 so the diff is confusing (come on GitHub please support upstream branches!!!!). Just look at this comparison.

[dominickng]

Does it make sense to surface a warning when processing the manifest that the current document URL is out of scope? Otherwise this seems good to me and I think Chrome can align with this behaviour.

[mgiuca]

I think if you want to consider this a legitimate use case (as you have argued to me), then we shouldn't show a warning.

If we consider it "bad enough" to show a warning, then I think we should just ban it.

[dominickng]

This is referring to the change to ban manifests from applying to out of scope documents. The ability to install from an out of scope document to an in-scope document on the same origin wouldn't be what the warning is targeting. But I'm fine with not surfacing a warning in any case.

[mgiuca]

This is referring to the change to ban manifests from applying to out of scope documents. The ability to install from an out of scope document to an in-scope document on the same origin wouldn't be what the warning is targeting. But I'm fine with not surfacing a warning in any case.

Hmm, I see. But there would be no way to distinguish a manifest that is designed to be applied for styling (which the warning is intended to target) and one that's intended for installation (which these days is by far the most common usage of Manifest). So there would just be a lot of false warnings to discourage something which we are actually supportive of.

[marcoscaceres]

@mgiuca, what should we do about this PR?

[mgiuca]

Hmm, I forgot about this. It might be superseded by #880. I'm waiting for your other PR to land before I tackle #880, so let me get back to this once those land.

[marcoscaceres]

sounds like a plan.