switch to unitless line-heights

This change fixes a [bizarre line-spacing problem](http://stackoverflow.com/questions/20695333/in-css-why-does-decimal-line-height-behave-differently-from-percentage-or-em) that crops up when CSS line-height is specified with a percentage or an em value, and the paragraph contains mutiple font sizes. ([Illustrated here](http://jsfiddle.net/Y7Jta/2/).) The fix is to use unitless line-height values.