Spec section: https://drafts.csswg.org/css-overflow-3/#overflow-propagation

This spec text confuses me:

UAs must apply the overflow-* values set on the root element to the viewport

As far as I understand, the viewport is not the root scroll container; the viewport is merely the scrollport of the root scroll container. So I don’t understand why something like html { overflow: scroll } should propagate to the viewport. Should it not propagate to the root scroll container instead? The overflow property applies to block containers, and the viewport is not a block container. The root scroll container is a block container, I assume.

Could you clarify this?