Because there is here as you mentionned Oauth.io server, but also Github server and lots of other servers for fonts/librairies etc...
Nothing is technically "serverless" on the web ;)
Client-flow is the far more interesting flow, for me, anyway. There the browser client gets the token directly from the auth server, that really is serverless. The key advantages of client-flow are you don't need the immediate server (less cost for you) and you don't have to see the data if you're using a third-party storage (better security and privacy).
You do need to be careful with security issues around the auth token itself, in server-flow you store the token on the server and have the session bind you to that token. Stealing a session is harder than stealing a token.
I know of two implementations to date:
Google's Official JavaScript Client library [0]
Dropbox's Unofficial JavaScript library [1]
https://github.com/voltagex/python-gist
Edit: Yes, I can. Until I get 100 users of the script, then I'd have to shut it down.
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