[webauthn] lowercase form of this RP ID

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

== lowercase form of this RP ID ==
You need to actually define what this means.

However, given that origins are already normalized, perhaps the 
algorithm you use (which you also don't seem to define) to derive the 
RP ID is already fine and "lowercase form" is just adding confusing.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/178 using your GitHub account