 CSS logical dimension

A CSS logical dimension measures a box relative to text flow rather than a fixed physical axis. The inline dimension is parallel to the flow within a line; the block dimension is perpendicular to it. CSS inline size and CSS block size are measurements along those respective dimensions.^[ref]

Let the inline size be $I$ and block size $B$. In a horizontal writing mode, physical width and height areIn a vertical writing mode, their roles exchange:These equations describe the dimension mapping; the box's actual used sizes still follow the applicable sizing and layout rules.^[ref]

For a simple definite-size example, take inline size $180$ and block size $60$ CSS pixels, with no padding, borders or conflicting constraints. The physical box is $180$ by $60$ in horizontal-tb, but $60$ by $180$ in vertical-rl or vertical-lr. The product $IB=10800$ is unchanged in this stipulated rectangle even though the physical width differs.

A direction reversal is a different operation. Changing CSS inline direction can exchange inline-start and inline-end while leaving the dimension itself the same. Likewise, vertical-rl and vertical-lr have the same inline/block axis mapping but different CSS block flow directions. An axis, its size and its directed start edge are related quantities with separate meanings.^[ref]

Logical dimensions help describe reusable layouts in terms such as the length available for a line and the extent available for successive lines or blocks. The specification's terms logical width and logical height refer to inline and block size respectively; they should not be read as promises about physical width and height in every mode.

This mapping also matters when a child uses a different writing mode from its containing context. Determine which box's CSS writing mode supplies each sizing reference before treating a number as a physical width. The dimension definitions provide the vocabulary, while the rules for the particular layout resolve its constraints.