Expected: nothing with a red background appears anywhere on this page. Every red-flagged node is light DOM that has no matching slot, so it stays in the DOM but is never laid out or painted.
Expected: box shows only "shadow only".
Expected: box shows "[kept]" — only the child whose slot name matches.
Expected: an unslotted element still takes no space — the two rules below are flush against the box with no gap for the 200px-tall hidden div.