DPR, Width, and Viewport-Width hints enable proactive content negotiation between client and server, enabling automated delivery of optimized assets - e.g. auto-negotiating image DPR resolution, image size, and other optimized assets based on signals such as client's viewport width. For more detail, see: https://docs.google.com/document/d/1SnRhnR_oWQ4Rivb7InJ9a_0T2k9CnuhNcMkd9xtLrKY/edit and https://github.com/igrigorik/http-client-hints#use-cases
Docs: https://github.com/igrigorik/http-client-hints https://developers.google.com/web/updates/2015/09/automating-resource-selection-with-client-hints https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation
Samples: https://github.com/igrigorik/http-client-hints#use-cases