[webauthn] `rpID` origin relaxation?

mikewest has just created a new issue for 
https://github.com/w3c/webauthn:

== `rpID` origin relaxation? ==
@annevk pointed out the algorithm in 
https://w3c.github.io/webauthn/#makeCredential which makes use of bits
 and pieces of `document.domain` that I would dearly love to remove 
from the platform. :)

It doesn't look like this (or the corresponding bits of 
`getAssertion()`) intend to change the document's origin, but it's not
 clear to me what impact they do have. Is the intent to support 
sharing auth tokens cross-origin? If so, could you help me understand 
why the origin model fails to support the use cases y'all have in 
mind?

Thanks!

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/338 using your GitHub account