Not sure I understand your distinction, the screen is taller now, if my browser wants to render to the top of the screen it has to contend with the notch.
If your browser changes nothing, the top bar will be blacked out and your tabs will display exactly as before. This only changes if your browser explicitly updates to handle the notched display.
It sounds like they're saying the OS will automatically add a black bar to the top in fullscreen mode, which will look seamless thanks to the contrast rate of mini LED.
If that is the case, it sounds like a pretty elegant solution and significantly reduces my concern about the notch. It also addresses my confusion about some of the images on Apple's website.
