[xvello]

One of the points of HTMX is to merge back all of the state to the backend, instead of spreading it between backend and frontend.

For authenticated users, just store a user pref record on your database. For state that is not linked to a user account, use the standard session mechanism of your backend. Alternatively, just use good-old functional cookies, with the Set-Cookie HTTP header. Set them to be available to javascript if needed.