I’ve been reading about web typography and came across a post by Erik Meyer, “Unitless line-heights”.

When you set the line height with a united value, the value will be computed and passed down to children. For instance, setting the line height to 1.5em for an element with font size of 16 points, means that the line height will be 24 points.

This value will be passed down to any element under the hierarchy, not the raw value of 1.5em. Meaning that even a descendant with 12 points will get a line height of 24 points.

Instead, using a unitless line height (like 1.5) will make sure that each element will compute the line height to be 1.5 times their font. This is preferable since line heights are very much font size dependent.