x-height alignment

Two faces set at the same nominal size rarely look the same size. Apparent size is x-height, not em: the em square is an arbitrary container, and where a designer put the lowercase letters inside it varies by hundreds of units between families. Set Inter and Equinor both at 16px and Equinor reads smaller — not because it is smaller, but because 48% of its em is lowercase against Inter's 55%.

This skill turns that into one measured number, recorded where a build can check it.

What you need before starting