What is the best design for unittes UI pages?

I have used many approaches in .Net Framework WebForms,Mvc,Angular and so on. But non of them do not propose simple way to test UI pages logic.

Lets imagine simple web page where we have two comboboxes country and state. When country change selected value state combobox should change its content and have appropriate states.

In my opinion the best solution is to have component which already contais this two comboboxes and server side code. At least we could test this component once manualy and then use it several times at project. But is is not universal and flexible solution.

What is the best design of such web UI to have clear and simple unit test of this scenorio ?