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SassMeister-input-HTML.html

<a class="blue-link"><span>H</span>ello</a>

SassMeister-input.scss

// ----
// Sass (v3.4.9)
// Compass (v1.0.1)
// ----

$blue: #00f;

@mixin hover-lighten {
  span {
    // currentColor works on its own
    background: currentColor;
  }
  
  &:hover {
    text-decoration: underline;
    //currentColor breaks in a function
    color: lighten(currentColor, 10%);
  }
}

.blue-link {
  @include hover-lighten;
  color: #00f;
}

SassMeister-output.css

$color: "currentColor" is not a color for `lighten'

SassMeister-rendered.html

<a class="blue-link"><span>H</span>ello</a>