Although I do not currently plan on adding per-site User-Agent switching, it would still be very helpful if you could outline some details on how you would expect such a feature to interact with the rest of User-Agent switcher. I'd be particularily interested in UI mockups and user stories (short step-by-step descriptions on what exactly you would expect to do [in terms of interaction with the extension] in order to achieve your goal). Please add any feedback to the existing issue: https://gitlab.com/alexander255/user-agent-switcher/issues/4
