Dividing features across different types of policies

[annevk]

As per https://github.com/w3c/webappsec-feature-policy/issues/282#issuecomment-486267212 the idea is to divide features across different policies. Here's an initial take on that using https://github.com/w3c/webappsec-feature-policy/blob/master/features.md as source:

Existing features

Feature name	Delegating permissions	Document sandboxing	Full sandboxing
accelerometer	✓
ambient-light-sensor	✓
autoplay	?
camera	✓
document-domain		? (I think this is a new primitive and I'm not convinced it's worth it)	✓
fullscreen	✓
gyroscope	✓
magnetometer	✓
microphone	✓
midi	✓
picture-in-picture	✓
sync-xhr		✓	✓
usb	✓
wake-lock	✓
xr	✓

Proposed features

Feature name	Delegating permissions	Document sandboxing	Full sandboxing
Client Hints
encrypted-media	?
geolocation	✓
payment	✓
speaker	✓

Experimental features

Feature name	Delegating permissions	Document sandboxing	Full sandboxing
document-write		✓	✓
font-display-late-swap		✓	✓
layout-animations		✓	✓
lazyload		✓	✓
legacy-image-formats		✓	✓
oversized-images		✓	✓
sync-script		✓	✓
unoptimized-images		✓	✓
unsized-media		✓	✓
vertical-scroll		✓	✓
serial

(I'm not entirely sure about all of these, let's try to keep this comment updated as we reach firmer conclusions on each of them.)

[eeeps]

Will these different feature-types be getting different:

• Syntaxes for values? (seems likely)
• Header/attribute names? (not sure)
• Specs? (seems less likely)

[clelland]

The most compelling insight from this table, I think, is that delegation and sandboxing are two different domains, and that if we're looking at splitting FP into different mechanisms, that seems like a clear distinction that we can make.

Delegation matches the existing spec quite well, and if we were to focus it on just that use case, we could probably trim it down a bit, too.

I'm just working on an actual proposal, but I would suggest:

• Different syntaxes, due to the deployment of existing delegation
• Different header / attribute names for the two kinds of features.
• One spec or two? Hard to say what's better; keeping everything here is administratively easier, certainly.

[clelland]

Closing with the introduction of Document Policy; the document/full sandboxing features listed in the initial description are all movable to that mechanism (I'll open a separate issue for SyncXHR)