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

About the [cloudinary use case](https://www.otsukare.info/2021/02/15/capping-macos-user-agent), this is indeed a real use case which has happened multiple times, where web developers will use the specific version number of an OS, or a browser or sometimes even a model to walk around a bug, **but this has a cost too**.

The optics of Cloudinary is I have a bug to fix with one browser/one OS not doing the right thing at a point in time, and I need to provide an alternate way for the users. And a well maintained Cloudinary providing a Web service to deliver to users works well. This is the poster child of a good use case. 

The optics of a browser vendors is there are thousands of websites using libraries that are locally deployed. These libraries have outdated strategies, bugs, and with logics such as:

* If `browser_x`, don't send this format (**even if the `browser_x` supports the format in the future**)
* If `browser_version` > `nn` sends [this] else sends [that] (even when  `nn+1` changes the strategy)
* no fallback, aka sites catering for most popular browsers.

The [Webcompat team at Mozilla](https://wiki.mozilla.org/Compatibility) is catering for this kind of issues relentlessly through the [webcompat.com](https://webcompat.com/). It takes time, it costs money and it creates a lot of frustration for users. 

I agree with @miketaylr that bad parsing has always been an issue, but better parsing will not solve bad and unmaintained logic. I understand @hsivonen about increasing the variability of surface (very similar topic than [Variability in Specifications](https://www.w3.org/TR/spec-variability/)) will probably increase the woes for browsers webcompat teams, issues which are affecting even more browsers with less market shares. 

And we can be sure about two things:

1.  that browser implementers will lie about the values provided in Client-Hints in the future because they have to for webcompat reasons
2. HTTP User Agent Strings will not go away because of legacy web properties. 



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