Box A
position
static
relative
z-index
auto
1
2
3
also
nothing
opacity: 0.99
transform
isolation: isolate
A-child z-index
1
5
9999
Box B
position
static
relative
z-index
auto
1
2
3
also
nothing
opacity: 0.99
transform
isolation: isolate
B-child z-index
1
5
9999
Layers are listed front to back. Boxes that are not envelopes do not appear as one item: their children join the page's own pile.