Re: [w3ctag/design-reviews] WebXR Layers (#528)

> It seems that exposing `nativeProjectionScaleFactor` will enable authors using the feature to obtain the scale factor for a given context (assuming post transform?). If this is the case, my assumption is that such capability will be very sought after many other places in the platform (users of CSSOM have been asking for something like this forever). Could this be either omitted or made available outside the WebXR context?

`nativeProjectionScaleFactor` is the value of [`scaleFactor`](https://immersive-web.github.io/layers/#dom-xrprojectionlayerinit-scalefactor) that was passed in during the construction of the projection layer.
It's basically the same as the [`framebufferscalefactor`](https://www.w3.org/TR/webxr/#dom-xrwebgllayerinit-framebufferscalefactor) value in the WebXR spec
It is NOT to the "scale factor for a given context (assuming post transform?)". WebXR Layers can only be used in immersive sessions so it has no relation with what is rendered on the page.

> Have you thought through some of the threat vectors that could abuse such capability? 

There's no new capability with the scale factor. The system will just allocate a buffer where the native buffer size is scaled by this number.

> How quickly and easily could a malicious actor create a WebXR context in a different content and obtain the composition layers?

It wouldn't be possible. Immersive WebXR sessions can only be created after a user action and consenting to go immersive. This is not something new that the WebXR Layers spec introduces; this is [already covered](https://www.w3.org/TR/webxr/#dom-xrsystem-requestsession) in the WebXR spec.

> One final meta question. Have you considered, or at least do you think it is feasible to break down this review into few logical pieces that build on top of each other? Layer Types, Spaces, Creation, Rendering etc. That model of reviews will be much more expedient to all of us.

As I mentioned earlier, this spec heavily depends on the WebXR specification so in order to review it, one has to be familiar with that one as well. 
I am planning on adding examples and non-normative explainers to clarify the intent. Right now it's mostly a list of interfaces and flow diagrams which makes it hard to follow.


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