The containment spec for contain:layout:

• says that contain:layout establishes a containing block for absolutely positioned and fixed positioned descendants, but
• does not say that it establishes a stacking context.

This is the first feature in CSS that I'm aware of with this combination. I think it's worth some consideration of whether this combination is sensible. Do the painting rules for abspos and fixpos elements contained by such an element work in a reasonable way?

/cc @MReschenberg