Save-DataSave-Data is a Client Hint request header telling the server that the user has enabled a 'Data Saver' mode in the browser or OS. It is sent only as on, and the server can respond with adaptive serving — lowering image quality or dropping heavy resources to reduce data usage.
The header behaves like a boolean: when saver mode is on, Save-Data: on is sent; when off, the header is omitted (no off value is sent). So the server judges by the header's presence. As a low-entropy hint it is sent by default without an Accept-CH opt-in.
The recommended response is to keep the core experience while cutting data cost: reduce image resolution/quality, increase lazy loading, suppress autoplay/prefetch/large fonts, and serve lower-resolution alternatives. The same signal is available in CSS via the prefers-reduced-data media query and in JS via navigator.connection.saveData.
If the response varies by Save-Data, you must send Vary: Save-Data so caches do not mix the saver and normal variants. The header respects the user's explicit wish to save data; it is not license to strip so much functionality that the site becomes half-broken.
Save-Data: <on>e.g. Save-Data: on
on | The user has enabled data-saving mode; if off, the header is not sent at all. |