But "stricter rules", apart from not being absolute enough to be strict, depend on fonts (including mostly invisible metrics like ascenders and descenders vs. l

But "stricter rules", apart from not being absolute enough to be strict, depend on fonts (including mostly invisible metrics like ascenders and descenders vs. line height) and on what kind of text is being spaced. For example, the visually important vertical rhythm of ink and gaps is dictated by line spacing, but in relationship with x-height (for mostly lowercase text), or capital height (for uppercase text) or something in between, and between lines in adjacent fonts depends on where the respective baselines are located.