Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="half">
  <div class="fluid-embed">
    <iframe width="560" height="315" src="https://www.youtube.com/embed/nfWlot6h_JM" frameborder="0" allowfullscreen></iframe>
  </div>  
</div>

SassMeister-input.scss

// ----
// libsass (v3.1.0)
// ----

$Private-Toolkit-Placeholders: (
);

//////////////////////////////
// Placeholder Get/Set
//////////////////////////////
@function toolkit-placeholder-set($id) {
  $unique-id: unique-id();
  $Private-Toolkit-Placeholders: map-merge($Private-Toolkit-Placeholders, ($id: $unique-id)) !global;
  @return $unique-id;
}

@function toolkit-placeholder-get($id) {
  @return map-get($Private-Toolkit-Placeholders, $id);
}

//////////////////////////////
// Generate Placeholder Class and Extend
//////////////////////////////
@mixin dynamic-extend($id, $extend: true) {
  $placeholder-id: toolkit-placeholder-get($id);

  @if $extend == true {
    @if $placeholder-id == null {
      $placeholder-id: toolkit-placeholder-set($id);
      @at-root %#{$placeholder-id} {
        @content;
      }
    }
    @extend %#{$placeholder-id};
  }
  @else {
    @content;
  }
}

@mixin button($color) {
  background: $color;
  
  @include dynamic-extend('button') {
    border: 1px solid;
		border-radius: 5px;
		padding: .25em .5em;
  }
}

.foo {
  @include button(red);
}

SassMeister-output.css

.foo {
  background: red;
  @at-root %uad2118cf {
    border: 1px solid;
    border-radius: 5px;
    padding: .25em .5em; }
 }

SassMeister-rendered.html

<div class="half">
  <div class="fluid-embed">
    <iframe width="560" height="315" src="https://www.youtube.com/embed/nfWlot6h_JM" frameborder="0" allowfullscreen></iframe>
  </div>  
</div>