The letter-spacing property is commonly used to make micro-adjustments to typography, so in practice you'd mostly see minuscule fractions such as 0.015em, or -0.05ems. Wouldn't the relative unit of 1/1000em that Adobe products use be more expressive for the vast majority of cases? A milliem (mem) of sorts, if you will. Other than personal experience (I've never tracked anything >= 1em), people seem to be asking about this: 1 2 3