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 Path: blob/main/files/en-us/web/http/headers/critical-ch/index.md
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---
title: Critical-CH
slug: Web/HTTP/Headers/Critical-CH
status:
  - experimental
browser-compat: http.headers.Critical-CH
---

{{HTTPSidebar}}{{SeeCompatTable}}{{SecureContext_Header}}

The Critical-CH client hint response header is used along with {{HttpHeader("Accept-CH")}} to specify that accepted client hints are also critical client hints.

User agents receiving a response with Critical-CH must check if the indicated critical headers were sent in the original request. If not, the user agent will retry the request along with the critical headers 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 following server configuration changes.

Each header listed in the Critical-CH header should also be present in the Accept-CH and Vary headers.

Header type {{Glossary("Response header")}}
{{Glossary("Forbidden header name")}} no

Syntax

Critical-CH: <ch-list>

Directives

  • <ch-list>

    • : A list of one or more comma-delimited client hint headers that the server considers to be critical client hints.

Examples

The client makes an initial request to the server:

GET / HTTP/1.1
Host: example.com

The server responds, telling the client via {{httpheader("Accept-CH")}} that it accepts {{httpheader("Sec-CH-Prefers-Reduced-Motion")}}. In this example Critical-CH is also used, indicating 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

Note: We've also specified Sec-CH-Prefers-Reduced-Motion in the {{httpheader("Vary")}} header to indicate that responses should be separately cached based on the value of this header (even if the URL stays the same). Each header listed in the Critical-CH header should also be present in the Accept-CH and Vary headers.

The client automatically retries the request (due to Critical-CH being specified above), 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"

The client will include the header in subsequent requests in the current session unless the Accept-CH changes in responses to indicate that it is no longer supported by the server.

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also