SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

/// Since the current way to qualify a class from within its ruleset is quite
/// ugly, here is a mixin providing a friendly API to do so.
/// @author Hugo Giraudel
/// @param {String} $element-selector - Element selector
@mixin qualify($element-selector) {
  @at-root {
    #{$element-selector + &} {
      @content;
    }
  }
}

/// @alias qualify
@mixin when-is($args...) {
  @include qualify($args...) { @content; }
}

.button {
  border: none;
  
  // Qualify `.button` with `button`
  @include qualify(button) {
    -webkit-appearance: none;
  }
  
  // Qualify `.button` with `a`
  @include when-is(a) {
    text-decoration: none;
  }
}

SassMeister-output.css

.button {
  border: none;
}
button.button {
  -webkit-appearance: none;
}
a.button {
  text-decoration: none;
}