But this isn’t about websites or just publishing articles or the weather. It’s about web applications, commerce, products, things with customers not just readers. As soon as you have an application that’s doing anything nontrivial then you have to draw a line and say “nope we’re not supporting that at all” and there’s sections along the line where you can’t even fail gracefully.

In the real world that means there is a list of browsers and versions tested on.