A unitless number is a CSS numeric value without a unit. Its meaning depends on the property receiving it: line-height treats 1.2 as a multiplier, while z-index and font-weight use numbers directly. A unitless 0 can replace a length or percentage value, but not an angle or time value. section 2.5
Unitless line-height is especially useful because the declared number is inherited and recalculated against each descendant’s local font size. With line-height: 1.2 and a 32px paragraph, the result is 38.4px. A unit-based value such as 1.2em can pass a computed length to children and cause overlapping text when their font sizes differ. section 2.5
Responsive paragraph spacing
Set body { line-height: 1.2; } and give .about-us { font-size: 2em; }. The paragraph inherits the multiplier, so its line height adapts locally instead of receiving a fixed computed length.
Design specifications can be converted into unitless line-height values: convert points to pixels by multiplying by 1.333, then divide by the font size. For example, 18pt becomes 24px, which becomes 1.5 with a 16px font. section 12.6.1
