[webauthn] Is _rpId_ supposed to look like an origin serialization, or like a hostname?

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

== Is _rpId_ supposed to look like an origin serialization, or like a 
hostname? ==
https://w3c.github.io/webauthn/#dom-webauthentication-makecredential 
step 3 has two branches.  the "rpId is not specified" branch will set 
_rpId_ to a string like "https://foo.bar.com".  The "rpId is 
specified" branch will set it to a string like "bar.com".

Is this difference purposeful?

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/259 using your GitHub account