Accept-CH

The Accept-CH HTTP header is an integral part of the Client Hints infrastructure, which allows servers to opt-in to specific client-provided information about the client’s environment. This information assists servers in optimizing resource delivery and adjusting to the specific requirements or constraints of the client’s environment.

The Purpose of Accept-CH

This response header field is used by a server to declare which Client Hints it is interested in receiving in future requests, both in the current network session and in subsequent sessions. These hints can be utilized to customize responses based on device or network conditions, leading to better performance and a superior user experience.

Syntax

The Accept-CH header uses the following syntax:

Accept-CH: <client-hint-token>

Here, <client-hint-token> represents one or more comma-separated tokens specifying the Client Hints that the server wants to receive. Each token corresponds to a specific type of client information, such as device memory, viewport width, or network connection type.

Possible Values

Here are a few examples of possible values for Accept-CH:

• DPR: Device pixel ratio
• Viewport-Width: Width of the client’s viewport in CSS pixels
• Width: Width of the requested resource in CSS pixels
• Save-Data: The client’s preference for reduced data usage

How it Works: Request and Response

Let’s illustrate the use of the Accept-CH header with a practical scenario:

Response

When a client first requests a resource, the server might respond with an Accept-CH header like this:

HTTP/1.1 200 OK
Accept-CH: DPR, Width, Viewport-Width

In this response, the server is expressing interest in the client’s device pixel ratio (DPR), the width of the requested resource (Width), and the width of the client’s viewport (Viewport-Width).

Request

Upon receiving this response, the client can choose to include these Client Hints in subsequent requests to the server. Here’s an example of what the next request from the client might look like:

GET /image.jpg HTTP/1.1
Host: example.com
DPR: 2.0
Width: 1920
Viewport-Width: 980

In this request, the client is sending the device pixel ratio, the width of the requested resource, and the width of its viewport. The server can use this information to better tailor the response to the client’s device and viewing conditions.

Compatibility

The Accept-CH header is not universally supported across all browsers and servers. For instance, Safari does not (yet) support Client Hints. Therefore, while using Accept-CH and related headers, it is important to design your services to degrade gracefully for clients that do not support or respect these hints.

Summary

The Accept-CH HTTP header is a powerful tool for enabling servers to customize their responses based on various aspects of the client’s environment, thus optimizing resource delivery. Although support for Client Hints is not universal, they can provide significant performance benefits when used properly. Always keep in mind both the server’s requirements and the client’s capabilities when implementing Accept-CH in your services.