public Effects fadeTo(double opacity, Function... f) {
    return fadeTo(Speed.DEFAULT, opacity, f);
  }
public Effects fadeTo ( double opacity , Function ... f ) { return fadeTo ( Speed . DEFAULT , opacity , f ) ; }
Fade the opacity of all matched elements to a specified opacity and firing
an optional callback after completion. Only the opacity is adjusted for
this animation, meaning that all of the matched elements should already
have some form of height and width associated with them.