example.styl

.3rdeden {
  background-image: url(gravatar('regular@address.com', 'fallback@address.com'))
}

// generates a valid gravatar that defaults to company e-mail and then a known github e-mail

mixin.styl

//
// Generates an MD5 hash from the data
//
md5(data)
  eval("require('crypto').createHash('md5').update('"+ data +"').digest('hex')")

//
// Generates valid
//
gravatar(regular, fallback, size=20)
  regular = md5(regular)
  fallback = md5(fallback)

  'https://secure.gravatar.com/avatar/'+ regular +'?s='+ size +'&d=https//secure.gravatar.com/avatar/'+ fallback +'?s='+ size

stylus.eval.js

stylus.define('eval', function evil(str) {
  return new stylus.nodes.String(eval(str.val));
});