I would consider myself mildly positive as to the direction, but that’s my 
personal view for the moment, absent consultation with my colleagues. I will 
solicit more viewpoints.

I particularly appreciate the responsiveness to feedback and that Yoav in 
particular has been willing to iterate.

I think there’s a number of things in the spec that should be cleaned up before 
an implementation ships enabled by default, specifically around interop, 
privacy, and protection against UA lockouts. I know there are PRs in flight for 
some of these issues. I think it would be good to get more of the open issues 
to resolution before actually shipping this.


> On May 7, 2020, at 4:22 PM, Michael Catanzaro <mcatanz...@gnome.org> wrote:
> 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.
> Michael
> [1] https://github.com/w3ctag/design-reviews/issues/467#issuecomment-583104002
> [2] https://mitm.watch/
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