Two Client Hints are in the works to provide exactly this. Check out Save-Data and Downlink: <a href="http:&#x2F;&#x2F;igrigorik.github.io&#x2F;http-client-hints&#x2F;" rel="nofollow">http:&#x2F;&#x2F;igrigorik.github.io&#x2F;http-client-hints&#x2F;</a>