Why would you need a custom selectbox in the example you provided? Because you could use the menu element with the popup API and AJAX if I understand your example correctly.
On a sidenote, do you know about HTMX? If so, what do you think about it?
On a sidenote, do you know about HTMX? If so, what do you think about it?
HTMX is the same story—it's what HTML could be, but isn't, unless monkey-patched in userland.