Declared line-height
The shaded band behind each sample is exactly as tall as the line boxes it holds - one line box in the single-line samples, and one per line in the wrapped ones further down. A declared line-height is the line box's height whether it is larger or smaller than the font's own ascent+descent (CSS 2.1 §10.8): when it is smaller the leading is negative and the glyphs deliberately overflow the band rather than the band growing to fit them.
One line, 24pt Georgia
line-height: 0.75 (shorter than the font - glyphs overflow the band)
Aligny jpqg
line-height: 1 (exactly the font-size)
Aligny jpqg
line-height: 1.5
Aligny jpqg
line-height: 36pt (absolute length)
Aligny jpqg
Several lines - every line box stacks by the declared height
line-height: 0.8 (lines pack tighter than the glyphs)
The quick brown fox jumps over the lazy dog, and then jumps back over it again for good measure.
line-height: 2
The quick brown fox jumps over the lazy dog, and then jumps back over it again for good measure.
The strut
Every line box is also tall enough for the block's own font and line-height - the strut (§10.8.1) - even when all of the line's actual content comes from an inline that declares a shorter one. Both bands below are the block's 30pt, not the inline's.
block line-height: 30pt, inline font: 6pt/6pt
a small inline on a tall block's line
block line-height: 30pt, inline line-height: 48pt (the taller inline wins instead)
Aligny