Pages intentionally submitting clean/empty frames

[alcooper91]

We recently encountered an issue in Chrome where one of the test pages was pumping frames (e.g. successfully requesting animation frames), but not actually drawing anything. As a result, Chrome ended up showing our immersive equivalent of the "sad tab" page. This may be partially covered by: https://immersive-web.github.io/webxr/#ref-for-dom-xrrenderstate-baselayer%E2%91%A7

This is easily fixed by simply drawing something to the page; but it does seem like there could be valid use cases (at least in AR), for not drawing content for a while (the system/UA is still drawing the real background so it's not like the user is staring at nothing. Content could either be loading or the user could be "searching" for something). We've discussed this somewhat in the past with #926 and #815, but I don't think we have concrete text one way or the other, so I think it's worth a new issue.

Should we allow (at least immersive-ar) sessions to submit empty frames without showing an error or should we treat this as an error?

/agenda