When users are on slow connections, Chrome will show an offline copy of a page if there is an offline version of the page available on the user’s device. Users will be shown a lite mode UI in the omnibox. If Chrome determines the network has 2G speeds and there's an offline page stored, Chrome shows the offline page. Chrome’s offline pages are based on MHTML snapshots. An intervention report is sent when this optimization occurs.
Docs: https://developers.google.com/web/updates/2018/09/reportingapi https://www.w3.org/TR/reporting/#intervention-report