This is just the intro. If you keep on with the following articles, they're about visual effects for stacked notification popups. They have good visuals, but if you have a stack of open notifications, something is wrong. If you have enough that you need a stack, users are going to want some way to put them in bins, order them, and generally manage them as a to-do list, like emails. At that point, they're not notifications, they're incoming messages.

No more than N% of the time should a notification be pending. Probably under 5%. Otherwise a notification is the wrong tool for the job. You need an inbox.