`font-size` in em depends on inherited `font-size`

Paragraph with `font-size` in em doesn't look good when inherited `font-size` is too big or too small: http://jsfiddle.net/2vJYE/3/

With `font-size` in px and `line-height` set equal to `height` result will not depend on inherited style: http://jsfiddle.net/2vJYE/2/