In CSS Box Alignment the alignment subject is the thing (or things) being aligned by the property.
- Block containers (including table cells)
The entire content of the block as a single unit.
- Multicol containers
The column boxes, with any spacing inserted between column boxes added to the relevant column gaps.
- Flex containers
- Grid containers
The grid tracks in the appropriate axis, with any spacing inserted between tracks added to the relevant gutters. Collapsed gutters are treated as a single opportunity for space insertion.