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.