// placeholder
@mixin optional-at-root($sel) {
  @at-root #{if(not &, $sel, selector-append(&, $sel))} {
    @content;
  }
}
@mixin placeholder {
  @include optional-at-root('::-webkit-input-placeholder') {
    @content;
  }
  @include optional-at-root(':-moz-placeholder') {
    @content;
  }
  @include optional-at-root('::-moz-placeholder') {
    @content;
  }
  @include optional-at-root(':-ms-input-placeholder') {
    @content;
  }
}
// usage placeholder
// @include placeholder {
//   properties for placeholder here
// }