My personal $0.02: I'm mildly supportive of this spec. It's certainly an improvement on existing HTTP user agent headers. I appreciate that you worked to incorporate feedback into the spec and considered the concerns of small browsers.

Is it going to solve all the problems caused by user agent headers? No. If WebKit implements the spec, we're surely going to eventually need a quirks list for user agent client hints to decide which websites to lie to, just like we already have quirks for the user agent header. And as long as Chrome sends a user agent header that includes the string "Chrome", it's unlikely we'll be able to get rid of the existing quirks list. But I think client hints will probably reduce the amount of websites that *accidentally* break WebKit, by replacing wild west UA header parsing with well-defined APIs, and adding some GREASE for good measure. The promise of freezing Chrome's UA header sounds nice, as it makes quirks easier to maintain. And being able to ration entropy by revealing details about the platform on an active rather than passive basis is quite appealing.

The spec attracted some misplaced concern about negative impact to small browsers, which I've rebutted in [1]. I'm not quite so enthusiastic about this spec as I was initially, especially after I was convinced that the GREASE is never going to be enough to remove our quirks list, but it's certainly not going to *hurt* small browsers.

This spec has received some pretty harsh criticism from the user tracking industry (some call it the "ad industry"). Not historically a friend of WebKit, so sounds good to me. ;)

One concern I haven't mentioned elsewhere is that frozen UA header might encourage deeper levels of fingerprinting than are currently used, e.g. for ad fraud prevention. caddy has started blocking WebKitGTK users based on TLS handshake fingerprint (yes, really!) [1]. If techniques like that take off as a result of this, that could potentially backfire on us quite badly. But websites could choose to do such things today anyway, client hints or no, and if so, the solution will be for us to just try even harder to look more like Chrome.

Seems like a net positive overall. I don't work for Apple and can't say whether it might be implemented by WebKit.



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