I think the real issue here isn't if JS control can or cannot (they certainly can) but that even if they can, they still wont behave like the native controls be

I think the real issue here isn't if JS control can or cannot (they certainly can) but that even if they can, they still wont behave like the native controls because the native controls depend on the native platform. E.g. imagine if someone is using a modified Gtk library that uses vi-like shortcuts/modes for multiline text areas. This wont work on custom JS controls. Or something more likely, someone using macOS which, AFAIK, has text edit controls that understand some Emacs-like shortcuts. Even if you sniff their OS from the user-agent or whatever and use macOS-like keys, the user still wont get the text functionality exposes via, e.g. the Services menu. Of course at the end of the day you may not care about such details - or care but you consider other things more important or have other concerns/limitations for the entire underlying system.