`line-height` [can and should be unit-less](https://developer.mozilla.org/en-US/docs/Web/CSS/line-height#Prefer_unitless_numbers_for_line-height_values). It helps avoid some [weird, unexpected cascading](http://meyerweb.com/eric/thoughts/2006/02/08/unitless-line-heights).

It’d be great to see units removed from `line-height` throughout Bitters, which would benefit more for avoiding this issue, but that’s for another PR…