Eric Meyer (who else?) has the answer: The property line-height can accept unitless number values. You can also give line-height united values, though generally you shouldn’t. But unitless numbers are just fine for this property. So what’s the difference? When you define a united value, like 1em, you’re setting things up to pass along the computed result to any descendants. … This is why it’s always strongly recommended that you use unitless numbers if you’re going to set a line-height on something like the html or body elements, or indeed on any element that is going to have descendant elements.