Unfinished sentence in HIG
In hig/C/in-app-notifications.page is a string that is unfinished/unclear:
"Allowing the user to undo a destructive action is an example of a good use of in-app notifications: the notification is not needed outside of the application window, and is immediately relevant. The button that allows the user to respond to the event, by pressing ."
The grammar in the last sentence seems a bit weird to me, and it doesn't mention which button should be pressed.