The Critical-CH header can trigger a request re-try. It's for situations where the browser could be unaware of the site's CH preferences (like the first navigation request to a site before the browser has received and stored CH preferences) or if a site has changed those references, and the site would rather drop the request and retry over getting a potentially "incomplete" request

This would *not* override potential mitigations or reductions in fingerprinting surfaces imposed by the browser. Any headers that would be blocked would still be silently dropped.

(cc davidben, mjs who I forgot to CC the first time)

On Thu, Jan 28, 2021 at 9:35 PM Ryosuke Niwa <rniwa@webkit.org> wrote:

What's the point of specifying Critical-CH as opposed to relying on CH provided by the browser?

Is the idea that some browsers may decide to hide some client hints to reduce the fingerprinting surface?

If so, then this new header seems to just defeat that because a website can specify all the client hints as critical.

- R. Niwa

On Wed, Jan 27, 2021 at 4:40 AM Aaron Tagliaboschi via webkit-dev <webkit-dev@lists.webkit.org> wrote:

Explainer: https://github.com/WICG/client-hints-infrastructure/blob/master/reliability.md#critical-ch

Draft Spec: https://tools.ietf.org/html/draft-davidben-http-client-hint-reliability-02#section-3

The Client Hint Reliability proposal is a set of features aimed at making Client Hints 

<https://tools.ietf.org/html/draft-ietf-httpbis-client-hints-15> more reliably available and mitigating 

mis-matches between a site's preferences and the preferences stored in the browser. The idea 

behind the Critical-CH response header is to signal to browsers that there are hints the server 

would rather pay a round trip than not have not the first request. The basic algorithm is as follows:

If, after receiving a request with Critical-CH and Accept-CH headers, there is a hint indicated in 

the Critical-CH header that the browser did not send but would not block sending, the browser 

should store the new CH preferences, drop the request, and start a new one with the new 

headers included.

Aaron Tagliaboschi | Software Engineer, Chrome Trust & Safety

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