Stylus REM mixin (font-size, line-height).

Stylus REM mixin

//define default values (best to put this in the default.styl file)  
base-font-size = 16
base-line-height = 24

//This is the useful part: it compares the given value to the base values and calculates the correct output
font-size($fontValue = base-font-size, $baseFontValue = base-font-size, $baseLineValue = base-line-height)
  font-size   $fontValue px
  font-size   ($fontValue / $baseFontValue) rem
  line-height ($fontValue / $baseFontValue) * $baseLineValue
  line-height ($baseLineValue/$baseFontValue) * ($fontValue / $baseFontValue) rem

//Test mixin, adjust the value to see it in action
p
    font-size(16)

Try it on the stylus website.

The result

p {
  font-size: 16 px;
  font-size: 1 rem;
  line-height: 24 px;
  line-height: 1.5 rem;
}