isso/docs/_static/css/bourbon/css3/_placeholder.scss

$placeholders: '-webkit-input-placeholder',
               '-moz-placeholder',
               '-ms-input-placeholder';

@mixin placeholder {
  @each $placeholder in $placeholders {
    @if $placeholder == "-webkit-input-placeholder" {
      &::#{$placeholder} {
        @content;
      }
    }
    @else if $placeholder == "-moz-placeholder" {
      // FF 18-
      &:#{$placeholder} {
        @content;
      }

      // FF 19+
      &::#{$placeholder} {
        @content;
      }
    }
    @else {
      &:#{$placeholder} {
        @content;
      }
    }
  }
}