[css-overflow-3] Overflow Viewport Propagation does not seem accurate

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?

Start with the CSS Overflow Level 3 overflow-propagation section linked in the issue, then compare its wording about the root element, viewport, scrollport, and root scroll container. Clarify the terminology and propagation target in the specification, with the section text itself providing the completion check.