A unitless number has no length or measurement unit; CSS interprets it according to the property. Its most useful typography role is preserving a proportional line-height relationship through inheritance. section 3.4.6
Why unitless line-height survives inheritance
With a parent font size of 20px and line-height: 150%, the child inherits a computed 30px line height. With line-height: 1.5, the child keeps the multiplier and computes 24px from its own 16px font size. The number 1.5 therefore scales with each descendant’s font size. section 3.4.6
Unitless values can also support responsive typography calculations. CSS custom properties store base font and line-height numbers, then calc() derives a unitless line-height and a pixel max-width for a chosen font size. section 8.1.2
Scaling type with a unitless factor
Starting from a 16px base font, a 1.45 base line-height, and a 24px article font size, the calculation produces line-height: 1.37 and max-width: 720px. Other font sizes produce corresponding values such as 1.49 at 12px and 1.29 at 32px. section 8.1.2
Outside CSS properties, a unitless value can represent an angle in radians. JavaScript trigonometric functions require radians, so degrees must be multiplied by Math.PI / 180 before calling Math.sin(), Math.cos(), or related functions. section 9.1.3
