We’ve been exploring what it would take to bring Device Sync to Web - the issue is that Realm is traditionally stored on disk and we don’t have access to that in a browser. We’ve been experimenting but it would be a long-term project. Your best bet now is to use the Data API, GraphQL, or HTTPS endpoints.
No - for Electron and React Native we commit data on disk in the realm file format. We are exploring IndexedDB and localstorage - they are not too amenable to an object database file format but we are researching options.