[css-contain-2] Spec is unclear about whether 0-sized boxes can be "relevant to the user" (since they have zero intersection area)

The "relevant to the user" ( https://www.w3.org/TR/css-contain-2/#relevant-to-the-user ) condition is defined as-follows:

An element is relevant to the user if any of the following conditions are true:

• The element is "on-screen": its containing box's border box intersects with the viewport, or a user-agent defined margin around the viewport.
• [...other conditions...]

This definition is ambiguous about whether or not a on-screen 0-sized box would be considered relevant or not. Such a box has 0 area, and hence it would have zero intersection, even if it's positioned on-screen. So it arguably never "intersects with the viewport" (i.e. it never has a nonzero intersection area), even when it is fully contained within the viewport.

It would be nice for the spec to make clear what happens for such boxes. Possibly just adding a sentence saying "For the purpose of this consideration, a 0-sized box is considered as intersecting this area if it is fully contained within this area."

Start with the CSS Containment Level 2 “relevant to the user” definition linked in the issue, then read the existing comment thread for prior discussion. Determine the intended treatment of fully contained 0-sized boxes and update the specification wording so the behavior is unambiguous.