Save-Data network client hint request header field is a boolean which indicates the client's preference for reduced data usage.
This could be for reasons such as high transfer costs, slow connection speeds, etc.
Save-Data is a low entropy hint, and hence may be sent by the client even if not requested by the server using an
Accept-CH response header.
Further, it should be used to reduce data sent to the client irrespective of the values of other client hints that indicate network capability, like
A value of
On indicates explicit user opt-in into a reduced data usage
mode on the client, and when communicated to origins allows them to deliver alternative
content to reduce the data downloaded such as smaller image and video resources,
different markup and styling, disabled polling and automatic updates, and so on.
Note: Disabling HTTP/2 Server Push (RFC 7540, section 8.2: Server Push) might be desirable too for reducing data downloads.
A value indicating whether the client wants to opt in to reduced data usage mode.
onindicates yes, while
off(the default) indicates no.
Vary header ensures that the content is cached properly (for
instance ensuring that the user is not served a lower-quality image from the cache when
Save-Data header is no longer present [e.g. after having switched from cellular to Wi-Fi]).
GET /image.jpg HTTP/1.0 Host: example.com Save-Data: on
HTTP/1.0 200 OK Content-Length: 102832 Vary: Accept-Encoding, Save-Data Cache-Control: public, max-age=31536000 Content-Type: image/jpeg […]
GET /image.jpg HTTP/1.0 Host: example.com
HTTP/1.0 200 OK Content-Length: 481770 Vary: Accept-Encoding, Save-Data Cache-Control: public, max-age=31536000 Content-Type: image/jpeg […]
|Save Data API |
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Varyheader which indicates that the content served varies depending on the value of
Save-Data(see HTTP Caching > Varying responses)
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- Improving user privacy and developer experience with User-Agent Client Hints (web.dev)