Tweak intro language
resource -> origin server

alternative services explicitly operate at the level of an
origin, not a resource.
available
s/will/can/

They aren't required to change it.
Add a nice little bridging para.
Just a tinier bit more clear
This seems to fit a bit better down here.
It's more than "can"; it's required.
s/needed/required/
Clarify a bit
Clarify request blocking non-requirement
clarify fail fallback

Can also be an alternative
Extraneous "however"
Move text connected to an example next to it.
Clarify frame directionality
s/server/service/
link to ref, not "see above"
This goes with Alt-Svc-Used, right?
Make introduction less passive
Work on "Using HTTP URIs.."