// placeholder mixin
.placeholder (@el:~'[placeholder]') {
  @{el}::-webkit-input-placeholder { .styles; }
  @{el}:-ms-input-placeholder { .styles; }
  @{el}::input-placeholder { .styles; }
  @{el}:input-placeholder { .styles; }
  @{el}::-moz-placeholder { .styles; }
  @{el}:-moz-placeholder { .styles; }
  @{el}::placeholder { .styles; }
  @{el}:placeholder { .styles; }
}