Em

A unit equal to the current type size: at 16 point, one em is 16 points. Measurement that scales with the type it measures.

The em is typography's relative ruler, named for the width of the capital M in metal days. Its virtue is proportionality: an indent of one em stays visually correct whether the type is 9 point or 90, because the unit grows with the letters it serves.

Screens inherited it whole: the CSS em and its steadier cousin the rem size interfaces relative to type, so layouts breathe with the reader's settings instead of fighting them. Spacing declared in ems is a design that carries its own proportions; spacing in fixed pixels is a design that trusts one size forever.