Original text:
There are thousands of different devices accessing the web, each with different device capabilities and preference information.  These device capabilities include hardware and software characteristics, as <del> well as dynamic user and client preferences.  Applications that want to allow the server to optimize content delivery and user experience based on such capabilities have, historically, had to rely on passive identification (e.g., by matching User-Agent (Section 5.5.3 of RFC7231) header field against an established database of client signatures), used HTTP cookies RFC6265 and URL parameters, or use some combination of these and similar mechanisms to enable ad hoc content negotiation. Such techniques are expensive to setup and maintain, are not portable across both applications and servers, and make it hard to reason for both client and server about which data is required and is in use during the negotiation: </del>  User agent detection cannot reliably identify all static variables, cannot infer dynamic client preferences, requires

Reviewer comments:
LGTM! thx
Some of this is very nit-picky; feel free to do what you like with it... ^ There's something weird going on with tense in this sentence. "Such techniques are expensive... are not portable... and make it hard..." The third doesn't fit with the first two. "Client Hints mitigate the performance concerns..." probably just "Client Hints mitigate performance concerns..." Likewise for "the privacy concerns" below. "This document defines the Client Hints infrastructure, a framework that enables servers to opt-in to specific proactive content negotiation features, which will enable them to adapt their content accordingly." --> "This document defines Client Hints, a framework that enables servers to opt-in to specific proactive content negotiation features, adapting their content accordingly." as a normative reference probably isn't going to fly, given its status. Why is it normative? maybe instead: Be aware that HTTP doesn't require presence of when proactive conneg is in use; if the response is truly uncacheable, it isn't necessary. maybe instead: instead: BTW, nothing is said about precedence between the two different mechanisms; separate issue? The reference to 6454 is spurious, and it's pretty indirect. Perhaps: It's kind of weird to have this list just appear. Perhaps turn these into prose? Put "For example" above the example to introduce it. Limits CH to just "optimisation." Probably should just be:
Thanks for reviewing! :) PR underway The here is conditional on cacheability. Is that OK? We're now considering removing http-equiv support for this: URL Maybe it's best to simply remove that reference here?
Ah, I missed that, sorry. WRT http-equiv; that might be best, then.

Revised text:
There are thousands of different devices accessing the web, each with different device capabilities and preference information.  These device capabilities include hardware and software characteristics, as  <ins> well as dynamic user and client preferences.  Historically, applications that wanted to allow the server to optimize content delivery and user experience based on such capabilities had to rely on passive identification (e.g., by matching User-Agent (Section 5.5.3 of RFC7231) header field against an established database of client signatures), used HTTP cookies RFC6265 and URL parameters, or use some combination of these and similar mechanisms to enable ad hoc content negotiation. Such techniques are expensive to setup and maintain, and are not portable across both applications and servers.  They also make it hard for both client and server to reason about which data is required and is in use during the negotiation: </ins> User agent detection cannot reliably identify all static variables, cannot infer dynamic client preferences, requires