//**
  Provides cross-browser css opacity.
  @param !opacity
    A number between 0 and 1, where 0 is transparent and 1 is opaque.
=opacity(!opacity)
  opacity= !opacity
  -moz-opacity= !opacity
  -khtml-opacity= !opacity
  -ms-filter= "progid:DXImageTransform.Microsoft.Alpha(Opacity=" + round(!opacity*100) + ")"
  filter= "alpha(opacity=" + round(!opacity*100) + ")"

// Make an element completely transparent.
=transparent
  +opacity(0)

// Make an element completely opaque.
=opaque
  +opacity(1)