I guess you are talking about user stylesheets and not user agents. Yes, they could but there's a lot of lightweight fonts you'd have to hardcode.
Also that's not possible on (most) mobile browsers and it shouldn't be something people have to do just to be able to read something in the first place.
Also that's not possible on (most) mobile browsers and it shouldn't be something people have to do just to be able to read something in the first place.