 CSS block size

CSS block size is a box's extent in the dimension perpendicular to text flow within a line. It corresponds to physical height in horizontal writing and physical width in vertical writing. The block-size property expresses that dimension without fixing one physical axis in its name.^[ref][ref]

For an own example, block-size: 80px; inline-size: 200px; describes a $200$-by-$80$ CSS pixel rectangle in horizontal-tb and an $80$-by-$200$ rectangle in vertical-lr, under simple definite-size assumptions. The latter has block progression toward the right, but the dimension's magnitude remains eighty pixels.

Box edges matter. With box-sizing: content-box; block-size: 20px;, ten-pixel padding and two-pixel borders on both block sides give an outer block extent ofpixels. That outer extent is physical height in a horizontal writing mode and physical width in a vertical one. CSS box-sizing and the applicable constraints determine how the declared value relates to the content and border boxes.

The declaration height: 50px; block-size: 80px; also changes meaning with writing mode. In horizontal writing, both address physical height, so the later block-size wins when their other cascade priorities are equal. In vertical writing, height constrains the inline dimension while block-size constrains physical width; both values can apply independently.^[ref]

CSS block flow direction identifies the direction in which blocks progress, whereas block size is a measurement along that axis. Moving from vertical-rl to vertical-lr changes the progression direction without swapping block size back to physical height.

An automatic block size can depend on content and layout, while min/max sizing properties can constrain the result. The useful interpretation is therefore an axis-relative size with the box-model and layout assumptions stated, not a universal promise that one declaration always fixes the painted rectangle exactly.