An inverted webapp architecture:

The webapp is statically hosted from a given domain.

A user can choose to plug in backend servers of their choosing into the webapp.

The webapp stores locally which backend servers the user has configured.

The primary domain doesn't do anything special or have any special permissions.

The domain maintains no state on the server, just statically serving up unchanging assets.

This pattern feels foreign in the same-origin paradigm, but it's a totally legal, just a bit atypical, pattern.

This pattern is great for things where different users might trust different commodity backends, or where there might be an ecosystem of different providers to plug in, each with a different catalog of content.