How can I create an efficient placeholder mixin in SCSS/CSS that handles nested and unnested selectors?

Placeholder Mixin in SCSS/CSS

When creating a SASS mixin for placeholders, a simple mixin like the one below might not suffice due to the number of colons and semi-colons that are passed through to it:

<code class="scss">@mixin placeholder ($css) {
  ::-webkit-input-placeholder {$css}
  :-moz-placeholder           {$css}
  ::-moz-placeholder          {$css}
  :-ms-input-placeholder      {$css}  
}</code>

Using @content Directive

To pass static CSS through to a mixin exactly as desired, the @content directive can be used. Here's an example:

<code class="scss">@mixin placeholder {
  ::-webkit-input-placeholder {@content}
  :-moz-placeholder           {@content}
  ::-moz-placeholder          {@content}
  :-ms-input-placeholder      {@content}  
}

@include placeholder {
    font-style:italic;
    color: white;
    font-weight:100;
}</code>

More information can be found in the SASS Reference at https://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#mixin-content.

Optional @-root Mixin (Sass 3.4 and above)

In Sass 3.4 and above, the optional-at-root mixin can be used to handle nested and unnested selectors effectively:

<code class="scss">@mixin optional-at-root($sel) {
  @at-root #{if(not &amp;, $sel, selector-append(&amp;, $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;
  }
}</code>

Usage:

<code class="scss">.foo {
  @include placeholder {
    color: green;
  }
}

@include placeholder {
  color: red;
}</code>

Output:

<code class="css">.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;
}</code>

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