Things it would be nice to get clarification on:

1. Should top layer elements "escape" ancestor visibility? Currently they escape ancestor opacity/masks/overflow. Are there other properties they should escape? From the current spec, what "etc." is isn't clear:

Ancestor elements with overflow, opacity, masks, etc. cannot affect it.

2. What happens when the ancestor is replaced content or display: table-column? See the following tests:

   • modal-dialog-in-replaced-renderer.html
   • modal-dialog-in-table-column.html
   • Currently those tests only pass in Chrome, and fail in both Safari/Firefox, so it'd be nice to resolve on what the expected behavior is here. (cc @emilio who mentioned this in dialog & ::backdrop web-platform-tests/interop#12 (comment))

3. Should top layer elements "escape" ancestor pointer-events? In all 3 current implementations the answer is no, ancestor pointer-events propagate to <dialog>, would be nice to make that clear!

4. Not sure "If it consists of multiple layout boxes, the first box is used." should be kept, this currently isn't tested, and is quite vague.

Personally, I don't find "It is rendered as an atomic unit as if it were a sibling of its root." very useful. The stacking context/containing block definitions sort of cover most things IMO.

Spec: https://fullscreen.spec.whatwg.org/#new-stacking-layer