If a Data Saver user is on a 2G-speed or slower network according to the NetInfo API, Chrome replaces images with placeholders and sends an intervention header on every resource request. Placeholders are the same dimensions as the original image to maintain site layout. Users are shown a Lite Mode UI in the omnibox. Users can load the image by long-pressing on the placeholder, or load all images by tapping “Show original” in the UI. An intervention report is sent when this optimization occurs.
Docs: https://wicg.github.io/netinfo/#dfn-effective-connection-types https://developers.google.com/web/updates/2018/09/reportingapi https://www.w3.org/TR/reporting/#intervention-report