This document defines a new request Client Hint header field, "CH", that allows the client to make available hints, both static and dynamic, to origin and intermediate servers about its preference and capabilities. "CH" allows server-side content adaption without imposing additional latency on the client, requiring the use of additional device databases, while allowing cache-friendly deployments.

This is such an obvious thing to do I've always wondered why it wasn't already implemented. At least it's being worked on. The idea here, if you can believe it, is when a URI is requested from a server, the server has very little information about the requesting client, and the information it does get (mainly, the "User-Agent" string), is difficult to convert into anything reliable or helpful. As URIs have transitioned away from being static documents towards more dynamic applications, our client browsers have multiplied along with device screen sizes and screen pixel densities. To serve an acceptable experience to the myriad of device types, we need to know more about the device which initiated the request. This is exactly what Client Hints hopes to accomplish. Good luck!