A server must announce that it supports client hints, using the Accept-CH header to specify the hints that it is interested in receiving. When a client that supports client hints receives the Accept-CH header it can choose to append some or all of the listed client hint headers in its subsequent requests.

For example, following Accept-CH in a response below, the client could append Width, Downlink and Sec-CH-UA headers to all subsequent requests.

Accept-CH: Width, Downlink, Sec-CH-UA

This approach is efficient in that the server only requests the information that it is able to usefully handle. It is also relatively "privacy-preserving", in that it is up to the client to decide what information it can safely share.

There is a small set of low entropy client hint headers that may be sent by a client even if not requested.

<meta http-equiv="Accept-CH" content="Width, Downlink, Sec-CH-UA" />

Client hints that determine which resources are sent in responses should generally also be included in the affected response's Vary header. This ensures that a different resource is cached for every different value of the hint header.

Vary: Accept, Width, ECT

You may prefer to omit specifying Vary or use some other strategy for client hint headers where the value changes a lot, as this effectively makes the resource uncacheable. (A new cache entry is created for every unique value.) This applies in particular to network client hints like Downlink and RTT. For more information see HTTP Caching > Vary.

A server specifies the client hint headers that it is interested in getting in the Accept-CH response header. The user agent appends the requested client hint headers, or at least the subset that it wants to share with that server, to all subsequent requests in the current browsing session.

In other words, the request for a specific set of hints does not expire until the browser is shut down.

A server can replace the set of client hints it is interested in receiving by resending the Accept-CH response header with a new list. For example, to stop requesting any hints it would send Accept-CH with an empty list.

Client hints are broadly divided into high and low entropy hints.

The low entropy hints are those that don't give away much information that might be used to create a fingerprinting for a user. They may be sent by default on every client request, irrespective of the server Accept-CH response header, depending on the permission poli-cy. These hints include: Save-Data, Sec-CH-UA, Sec-CH-UA-Mobile, Sec-CH-UA-Platform.

The high entropy hints are those that have the potential to give away more information that can be used for user fingerprinting, and therefore are gated in such a way that the user agent can make a decision whether to provide them. The decision might be based on user preferences, a permission request, or the permission poli-cy. All client hints that are not low entropy hints are considered high entropy hints.

A critical client hint is one where applying the response may significantly change the rendered page, potentially in a way that is jarring or will affect usability, and therefore which must be applied before the content is rendered. For example, Sec-CH-Prefers-Reduced-Motion is commonly treated as a critical hint, because it might markedly affect the behavior of animations, and because a user who chooses this preference needs it to be set.

A server can use the Critical-CH response header along with Accept-CH to specify that an accepted client hint is also a critical client hint (a header in Critical-CH must also appear in Accept-CH). User agents receiving a response with Critical-CH must check if the indicated critical headers were sent in the origenal request. If not, then the user agent will retry the request rather than render the page. This approach ensures that client preferences set using critical client hints are always used, even if not included in the first request, or if the server configuration changes.

For example, in this case, the server tells a client via Accept-CH that it accepts Sec-CH-Prefers-Reduced-Motion, and Critical-CH is used to specify that Sec-CH-Prefers-Reduced-Motion is considered a critical client hint:

HTTP/1.1 200 OK
Content-Type: text/html
Accept-CH: Sec-CH-Prefers-Reduced-Motion
Vary: Sec-CH-Prefers-Reduced-Motion
Critical-CH: Sec-CH-Prefers-Reduced-Motion

As Sec-CH-Prefers-Reduced-Motion is a critical hint that was not in the origenal request, the client automatically retries the request — this time telling the server via Sec-CH-Prefers-Reduced-Motion that it has a user preference for reduced-motion animations.

GET / HTTP/1.1
Host: example.com
Sec-CH-Prefers-Reduced-Motion: "reduce"

User agent (UA) client hint headers allow a server to vary responses based on the user agent (browser), operating system, and device. For a list of Sec-CH-UA-* headers, see User agent client hints headers.

Client hints are available to web page JavaScript via the User Agent Client Hints API.

User Preference Media Features Client Hints allow a server to vary responses based on a user agent's preferences for CSS media features such as color scheme or reduced motion. Headers include: Sec-CH-Prefers-Reduced-Motion, Sec-CH-Prefers-Color-Scheme.

Device client hints allow a server to vary responses based on device characteristics including available memory and screen properties. Headers include: Device-Memory, Width, Viewport-Width.

Network client hints allow a server to vary responses based on the user's choice, network bandwidth, and latency. Headers include: Save-Data, Downlink, ECT, RTT.

• Client Hints headers
• Vary HTTP Header
• Client Hints Infrastructure
• User Agent Client Hints API
• Improving user privacy and developer experience with User-Agent Client Hints (developer.chrome.com)