In a custom element, some children might not be assigned to any slot. For instance see: https://juliandescottes.github.io/webcomponents-playground/test-unslotted-light-dom/ It would be nice if the markup view could show the second <div> under <test-component> differently, since this node is not rendered in the tree. Maybe we could have a small warning icon, or render the node faded out (same as for invisible nodes).