But no matter how good or bad a particular browser is, you still have less work to do if you only care about users on a single browser (compared to that browser

But no matter how good or bad a particular browser is, you still have less work to do if you only care about users on a single browser (compared to that browser _and_ other browsers). Even if the other N-1 browsers have no quirks relative to the first one, that's still more testing to do and support to give. It's objectively easier to target fewer platforms, which is why you see the laziest web developers put up warnings (or even block requests) for clients with the "wrong" User-Agent strings.