Re: [w3ctag/design-reviews] User-Agent Client Hints & UA Reduction (#640)

> @yoavweiss can you provide any further detail or comment here? 

@yoavweiss is on PTO this week, but I’m happy to reply.

@jorabin-51d [writes](https://github.com/w3ctag/design-reviews/issues/640#issuecomment-992811213):
> We are therefore very concerned that the proposals will “break the Web” in various ways, and therefore require very close scrutiny…
> …The form in which changes have been proposed stand a good chance of “breaking the Web”.

I really appreciate the concern for compatibility - it was a guiding principle when we designed UA Reduction and its phased rollout. Most of the use cases mentioned above rely on hints that are available by default (user experience improvements, analytics, etc.) and will not be reduced in the User-Agent header. A site won’t need to migrate to UA-CH if they only rely on browser name, major version, platform, or mobileness. Use cases like fraud detection would require extra hints, and migrating to UA-CH will allow those to continue to function.

That said, we haven’t received concrete evidence of site compat issues through our own testing or from users who have enabled the chrome://flags/#reduce-user-agent flag. We’ve also been running an Origin Trial since Chrome 95 (launched on October 19th of last year) but have not received reports of site breakage, either publicly or through partner channels. If @jorabin-51d does know of sites that will break, we welcome reports at https://crbug.com/new so we can work with the sites to get them fixed.

@jorabin-51d [also writes](https://github.com/w3ctag/design-reviews/issues/640#issuecomment-1040143761):
> That change hasn’t been standardised…

We hope to bring UA-CH to the W3C (and [have already stated as much](https://github.com/w3ctag/design-reviews/issues/640#issuecomment-853634160) in this thread) once another implementer has stated an interest in implementing it.

Client Hints (which UA-CH builds upon) [is standardized in the IETF](https://datatracker.ietf.org/doc/html/rfc8942).

We recently started the work to document User-Agent Reduction patterns by Firefox, Chrome, and Safari [in the WHATWG](https://compat.spec.whatwg.org/#ua-string-section). There’s more work to be done there, and it’s on my plate over the coming months.

>  …and hasn’t yet had a TAG design review

I’ll also note that we’re having this discussion in the middle of a TAG design review. :)


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