The default line-height in most browsers (1.2) is almost universally too tight for body text. The conventional recommendation for comfortable reading is 1.4–1.6 for body text — enough space that the eye can track horizontally without confusing one line for the next, but not so much that lines feel disconnected. For wider text columns, more leading is needed; for narrower columns, less.
Leading interacts with type size, font choice, and column width in complex ways. A condensed typeface with tight letterforms needs more leading than a humanist typeface with open apertures. Long text columns need more leading than short ones. Display type at large sizes often looks better with tighter leading than body text — headlines at 0.9 line-height can feel powerful and editorial, while the same setting on body text would be unreadable.
Vertical rhythm — the use of a consistent leading increment across all text sizes in a layout — creates a visual grid in the time dimension. When headlines, subheadings, and body text all share a common leading unit (or multiples of it), the layout feels organised at a level below conscious awareness. This is the typographic equivalent of a horizontal grid.