Sec-CH-Prefers-Reduced-Transparency header
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.
The HTTP Sec-CH-Prefers-Reduced-Transparency 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 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 prefers-reduced-transparency media query.
| Header type | Request header, Client hint |
|---|---|
| Forbidden request header | Yes (Sec- prefix) |
Syntax
Sec-CH-Prefers-Reduced-Transparency: <preference>
Directives
<preference>-
The user agent's preference for reduced transparency. This is often taken from the underlying operating system's setting. The value of this directive can be either
no-preferenceorreduce.
Examples
Using Sec-CH-Prefers-Reduced-Transparency
The client makes an initial request to the server:
GET / HTTP/1.1
Host: example.com
The server responds, telling the client via Accept-CH that it accepts Sec-CH-Prefers-Reduced-Transparency. In this example 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-Transparency in the 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 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-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
| Specification |
|---|
| User Preference Media Features Client Hints Headers # sec-ch-prefers-reduced-transparency |
Browser compatibility
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Sec-CH-Prefers-Reduced-Transparency request header
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Legend
Tip: you can click/tap on a cell for more information.
- Full support
- Full support
- No support
- No support
- Experimental. Expect behavior to change in the future.
See also
- Client hints
- User-Agent Client Hints API
Accept-CH- HTTP Caching: Vary and the
Varyheader - Improving user privacy and developer experience with User-Agent Client Hints (developer.chrome.com)