The "remember your username" usually means persisting a user ID in the cookie session, and the core banking backend does not know shit about cookies or front-en

The "remember your username" usually means persisting a user ID in the cookie session, and the core banking backend does not know shit about cookies or front-end users, the request needs to be mediated and properly authenticated in a way that is compliant with security regulations. You could conceivably pre-render the backend results before hand but then on first load the visiting user would not see her latest transactions --which is a very frequent use case. As a side note, it's amazing how modern banking sites keep being really fancy interfaces to years old COBOL backends.