files/en-us/web/http/reference/headers/sec-ch-prefers-reduced-transparency/index.md
{{SeeCompatTable}}{{SecureContext_Header}}
The HTTP Sec-CH-Prefers-Reduced-Transparency {{Glossary("request header")}} is a user agent client hint that indicates the user agent's preference for reduced transparency.
If a server signals to a client via the {{httpheader("Accept-CH")}} header that it accepts Sec-CH-Prefers-Reduced-Transparency, the client can then respond with this header to indicate the user's preference for reduced transparency. The server can send the client appropriately adapted content — for example, CSS or images — to reduce the transparency of the content.
This header is modeled on the {{cssxref("@media/prefers-reduced-transparency", "prefers-reduced-transparency")}} media query.
<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td> {{Glossary("Request header")}}, <a href="/en-US/docs/Web/HTTP/Guides/Client_hints">Client hint</a> </td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>Yes (<code>Sec-</code> prefix)</td> </tr> </tbody> </table>Sec-CH-Prefers-Reduced-Transparency: <preference>
<preference>
no-preference or reduce.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 Sec-CH-Prefers-Reduced-Transparency. In this example {{httpheader("Critical-CH")}} is also used, indicating that Sec-CH-Prefers-Reduced-Transparency is considered a critical client hint.
HTTP/1.1 200 OK
Content-Type: text/html
Accept-CH: Sec-CH-Prefers-Reduced-Transparency
Vary: Sec-CH-Prefers-Reduced-Transparency
Critical-CH: Sec-CH-Prefers-Reduced-Transparency
[!NOTE] We've also specified
Sec-CH-Prefers-Reduced-Transparencyin the {{httpheader("Vary")}} header, to indicate to the browser that the served content will differ based on this header value — even if the URL stays the same — so the browser shouldn't just use an existing cached response and instead should cache this response separately. Each header listed in theCritical-CHheader should also be present in theAccept-CHandVaryheaders.
The client automatically retries the request (due to Critical-CH being specified above), telling the server via Sec-CH-Prefers-Reduced-Transparency that it has a user preference for reduced transparency:
GET / HTTP/1.1
Host: example.com
Sec-CH-Prefers-Reduced-Transparency: "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}}
{{Compat}}