What is a Save-Data header?
Tells the server the client has requested a reduced-data-usage mode, so lighter responses should be served where possible.
Save-Data lets a visitor on a slow or metered connection ask a website to go easy on the bandwidth - serving smaller images, skipping non-essential scripts, that kind of thing. Few sites actually implement anything in response to it today, but for a performance-conscious website it's a genuinely useful, low-effort signal to build lighter-weight response variants around.
Common use case
A media-heavy site checks for Save-Data: on and serves lower-resolution images or skips autoplaying video previews for visitors on a constrained connection.
Example
Save-Data: onHistory
Introduced by Google as part of the Client Hints proposal, first shipped in Chrome around 2016, and later formalised within the broader Client Hints specification.
Did you know?
It was originally built to support Chrome's Data Saver feature on Android, aimed specifically at visitors on expensive or limited mobile data plans in markets where that's a major concern.