> In the real DOM you need to know which element from project _from_. The <slot> itself is only half the connection. In a light-DOM only document full of <slot>s you won't know which elements to project where.

There's still a concept of "importing" a template. A host node has slots under it and requests to "import" a template from elsewhere in the document. That import process can adopt the slots then. It's "adoption" rather than "projection" and you "lose" that relation to the source document that "slots stay in the same place", but the DOM has never been static with respect to source documents.