Re: [w3ctag/design-reviews] `Accept-CH` header is weird (#206)

> So the alternative is that browsers send this data in all requests all the time? Since the data is relatively small and there's no user-opt-in, I see no obvious problem with that.

Loud and clear feedback from implementers to date it that they are **not willing** to do this. Exposing "unproven" headers on _every_ request is a very high bar and it doesn't scale, and the goal of CH is to establish a framework that can scale to (many) dozens of different hints.

As a result, we need an origin opt-in mechanism, which is what [`Accept-CH` defines](http://httpwg.org/http-extensions/client-hints.html#accept-ch): it's a response header that advertises which hints the origin supports, or is interested in. Also, as noted in #190, there is a companion [Accept-CH-Lifetime](http://httpwg.org/http-extensions/client-hints.html#accept-ch-lifetime) which persists this preference for specified time, allowing these hints to be delivered on subsequent navigations.

> perhaps Allow-CH (to align with the response nature of Allow)?

I'm not sure that `Allow` makes that any clearer to the uninitiated, and we do have Accept-CH being used in the wild already, so I'd be very hesitant about making this change unless it's a clear an unambiguous win.


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