//**
   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)