CSS block flow direction

The CSS block flow direction is the direction in which block-level boxes and successive line boxes progress in the relevant flow. The writing-mode property determines this direction: horizontal-tb progresses top to bottom, vertical-rl right to left, and vertical-lr left to right.^[ref]

For an own geometry example, take three ordinary in-flow block boxes with zero margins, borders and padding, each having inline size $100$ and block size $30$ CSS pixels. Assume no conflicting sizing constraints and a containing region sized to fit them. In horizontal-tb, the parent is $100$ pixels wide and $90$ high. The boxes occupy successive vertical intervals $[0,30]$, $[30,60]$ and $[60,90]$.

In vertical-rl, those same CSS logical dimensions give a parent $90$ pixels wide and $100$ high. Measuring physical $x$ from the left edge, the first box occupies $[60,90]$, the second $[30,60]$ and the third $[0,30]$. Their source order is unchanged, but the block progression runs toward decreasing physical $x$. These are stipulated box geometries, not font-dependent text measurements.^[ref]

CSS inline direction concerns progression within a line and its base directionality. Setting direction: rtl in an otherwise horizontal example does not make the block stack run upward: writing-mode still supplies its top-to-bottom block flow. Likewise, the rl in vertical-rl describes block progression rather than simply naming right-to-left text ordering.

This distinction also affects CSS scroll origin, whose default anchor uses block-start and inline-start. In the horizontal example, block-start is top; in the vertical right-to-left block flow it is right. Rules expressed using logical starts and sizes can therefore follow the layout convention instead of hard-coding one physical edge.^[ref]

A writing-mode change determines line orientation and block progression, while CSS glyph orientation determines how text is laid out inside those lines. Treating all three as a single rotation would lose these separate layout effects.