Ideally, imo, most sites should use the web platform as intended and their sites will work fine regardless of input device.

If you are using UA sniffing to determine if the device has a keyboard it will be wrong a lot of the time; you can’t tell if a Microsoft Surface has the keyboard attached or an iPad doesn't. you’d probably be no worse off trying to detect it by screen width, which is already a defacto way to detect mobile devices anyways, for better or worse.

(The right solution is probably a new kind of API or media query.)