Line height follows the measure
Leading and line length are one decision. This suggests a value from both — and says when the measure is the real problem.
Line height follows the measure
Leading and line length are one decision: a long line needs more space for the eye to find the start of the next one. This suggests a value from both, and says when the measure is the real problem.
Why unitless
line-height: 1.5 multiplies each element's own font size. line-height: 24px is inherited as a computed length, so a child with a 32px font size still gets 24px of leading and its lines collide.
That is the single most common line-height bug, and it only appears when someone nests a heading inside a container that set leading in a length.
WCAG 1.4.12 requires that content still works when a reader forces line height to at least 1.5× the font size. Testing with that override is more useful than picking a number and hoping.
More leading does not fix a long line
Past about 90 characters the eye loses the return sweep — the jump back to the start of the next line — and no amount of extra space repairs it. Narrowing the column does.
The classic range is 45 to 75 characters. Below 45 the rhythm breaks the other way, with too many returns and more hyphenation.
Larger text needs less
The return sweep is easier when the text is bigger, so display sizes want tighter leading — often 1.1 to 1.2 where body copy wants 1.5 to 1.7. A single line-height across a type scale is always wrong at one end.