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.