HTTP Client Hints (RFC 8942 and W3C draft) are a set of standardized HTTP request headers that allow web browsers to proactively provide servers with device, network, and user-preference details (such as viewport width, device memory, downlink speed, and dark mode preference). Because legacy User-Agent strings were heavily abused for passive browser fingerprinting, modern browsers are freezing User-Agent headers and replacing them with Client Hints, which require explicit server opt-in via Accept-CH.

Accept-CH informs the browser which hints the server wants on subsequent requests. However, on the very first navigation request to a website, the browser does not yet know the server's Accept-CH policy and sends standard default headers. If the server cannot render the first page correctly without specific hints (e.g. high-DPI layout or country-specific resources), the server responds with Critical-CH: Sec-CH-Viewport-Width, Sec-CH-DPR. The browser immediately retries the request with the required hints without user disruption.

Client Hints are strictly restricted to the first-party origin by default. A third-party CDN or image service cannot receive high-entropy hints (like exact device memory or precise viewport dimensions) unless the top-level document explicitly delegates them using the Permissions-Policy HTTP header (e.g., Permissions-Policy: ch-viewport-width=(self "https://cdn.imagecloud.com")).

Edge platforms (Cloudflare Workers, Fastly, AWS CloudFront) intercept headers like Sec-CH-ECT (Effective Connection Type: 4g, 3g, 2g) and Sec-CH-Downlink directly at the point-of-presence (PoP). If a client is detected on a 2G connection or with Save-Data: on, the edge worker automatically recompresses images from 4K PNG to 800px AVIF with aggressive quantization, deferring non-critical scripts and slashing page payload sizes by up to 90%.