Placeholder mixin for Sass

placeholder.scss

// Placeholder @mixin for Sass
//
// A mixin to style placeholders in HTML5 form elements.
// Includes also a .placeholder class to be used with a polyfill e.g.
// https://github.com/mathiasbynens/jquery-placeholder
// Requires Sass 3.2.
//
// Example usage (.scss):
// 
// input {
//   @include placeholder {
//     /* styles for placeholder here */
//   }
// }
//

@mixin placeholder {
  &.placeholder { @content }
  &:-moz-placeholder { @content }
  &::-webkit-input-placeholder { @content }
}

In Sass 3.4 you can use this snippet to work both nested and unnested rules

@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:

.foo {
  @include placeholder {
    color: green;
  }
}

@include placeholder {
  color: red;
}

Output:

.foo::-webkit-input-placeholder {
  color: green;
}
.foo:-moz-placeholder {
  color: green;
}
.foo::-moz-placeholder {
  color: green;
}
.foo:-ms-input-placeholder {
  color: green;
}

::-webkit-input-placeholder {
  color: red;
}
:-moz-placeholder {
  color: red;
}
::-moz-placeholder {
  color: red;
}
:-ms-input-placeholder {
  color: red;
}

From this question https://stackoverflow.com/a/17181946