Allows web apps (as defined by the Web App Manifest standard) to set an app-wide badge in operating-system-specific places such as the shelf or home screen. Additionally, it gives the app a small, visible place to notify the user of new activity that might require attention, without showing a full notification. It can show additional information, such as an unread count or event type. It allows the app to convey this information when its windows are closed.
Docs: https://web.dev/badging-api
Explainers: https://github.com/WICG/badging/blob/master/explainer.md