HTTP Client hints are a set of request headers that provide useful information about the client such as device type and network conditions, and allow servers to optimize what is served for those conditions.

Servers proactively requests the client hint headers they are interested in from the client using Accept-CH. The client may then choose to include the requested headers in subsequent requests.

Servers can advertise support for Client Hints using the Accept-CH header field or an equivalent HTML element with http-equiv attribute.

Servers use Critical-CH along with Accept-CH to specify that accepted client hints are also critical client hints.

The different categories of client hints are listed below.

The UA client hints are request headers that provide information about the user agent, the platform/architecture it is running on, and user preferences set on the user agent or platform:

User agent's branding and version.

User agent's underlying platform architecture.

User agent's underlying CPU architecture bitness (for example "64" bit).

Full version for each brand in the user agent's brand list.

User agent is running on a mobile device or, more generally, prefers a "mobile" user experience.

User agent's device model.

User agent's underlying operation system/platform.

User agent's underlying operation system version.

User's preference of dark or light color scheme.

User's preference to see fewer animations and content layout shifts.

Approximate amount of available client RAM memory. This is part of the Device Memory API.

Network client hints allow a server to choose what information is sent based on the user choice and network bandwidth and latency.

Approximate bandwidth of the client's connection to the server, in Mbps. This is part of the Network Information API.

The effective connection type ("network profile") that best matches the connection's latency and bandwidth. This is part of the Network Information API.

Application layer round trip time (RTT) in milliseconds, which includes the server processing time. This is part of the Network Information API.

A string on that indicates the user agent's preference for reduced data usage.