#opacity {
  background: alpha(rgba(#f28500, .5));
  background: opacity(rgba(#f28500, .5));
  background: opacify(rgba(#f28500, .2), .5);
  background: fade-in(rgba(#f28500, .2), .5);
  background: transparentize(#f28500, .5);
  background: fade-out(#f28500, .5);
}