[css-scoping] Explanation required : Why only direct children of the shadow host can be slotted?

[Praveen Puglia]

The Example 4 here says that

only direct children of a shadow host can be assigned to a slot.

I couldn't find any explanation as to why it's restricted to only direct children. Should it be mentioned in the spec somewhere as a note or a reference link which expands on it?

Or please tell me if I missed something totally obvious.

[Tab Atkins Jr.]

That's just part of how Shadow DOM is defined, over in the DOM spec. CSS doesn't impose this or care about it, I just called it out in the example to be informative.