Proposed API extension for Fido U2F devices

Hi folks,
I'm aware that hardware-backed keys are out of scope for the current round
of WebCrypto work, so I don't expect this to be ready for standardization
for some time. Nevertheless, I've got a proposed extension to WebCrypto to
support Fido Alliance (fidoalliance.org) universal second factor (U2F)
devices:
https://docs.google.com/a/chromium.org/document/d/1EEFAMIYNqZ7XHCntghD9meJwKgNOX7ZN-jl5LJQxOVQ/edit#

I apologize that the proposal may lack some context, like, just what is a
U2F device, and what language does it speak? I promise update it with
pointers to public docs once they are made public. In the meantime, I'll
act as a poor substitute by answering questions myself, either in the doc
or in email.

I'd appreciate any feedback you might have. Thanks very much,
--Juan