Sweet! Recall that mixins, unlike extended selectors, insert the code inside the selector's rules wherever they are included, only including "original" code if they are assigning a new value to the rule's properties via an argument.

Let's look at the @mixin and @extend constructs closely and compare output:

@mixin no-variable {
  font-size: 12px;
  color: #FFF;
  opacity: .9;
}

%placeholder {
  font-size: 12px;
  color: #FFF;
  opacity: .9;
}

span {
  @extend %placeholder;
}

div {
  @extend %placeholder;
}

p {
  @include no-variable;
}

h1 {
  @include no-variable;
}

would compile to:

span, div{
  font-size: 12px;
  color: #FFF;
  opacity: .9;
}

p {
  font-size: 12px;
  color: #FFF;
  opacity: .9;
  //rules specific to ps
}

h1 {
  font-size: 12px;
  color: #FFF;
  opacity: .9;
  //rules specific to ps
}

We can clearly see extending results in way cleaner and more efficient output with as little repetition as possible.

As a general rule of thumb, you should

• Try to only create mixins that take in an argument, otherwise you should extend.

• Always look at your CSS output to make sure your extend is behaving as you intended.