 CSS strut

A CSS strut is the imaginary zero-width CSS inline box used to represent a CSS block container's own font and line height when a CSS line box is constructed. It supplies minimum extents on both sides of the text baseline, even when the actual text uses smaller metrics.^[ref]

Suppose the container has font ascent $A=12$, font descent magnitude $D=4$ and line height $H=20$ CSS pixels. Its CSS leading is $20-12-4=4$, so the strut contributes 14 above and 6 below the text baseline. A small text contribution with extents 7.5 above and 2.5 below still produces a line 20 CSS pixels high because the strut dominates both directions.

The minimum is geometric rather than merely a comparison of total heights. If another contribution extends 18 above and 2 below the same text baseline, the enclosing CSS line box becomes $18+6=24$ CSS pixels. Both the strut and the other contribution individually total 20. Keeping the separate upper and lower extents explains the extra height.

This construction concerns a CSS line box that participates in layout. CSS also specifies conditions under which empty line boxes are treated as nonexistent, so line-height:20px alone does not make every otherwise empty block 20 CSS pixels high.^[ref]