For justify-self and align-self , the alignment subject is the margin box of the box the property is set on, using the writing mode of that box. For justify-content and align-content , the writing mode of the box is also used.

The definition of the alignment subject depends on the layout mode being used.

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

For justify-content , the flex items in each flex line. For align-content , the flex lines. Note, this only has an effect on multi-line 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.