0_usage.scss

@include keyframes(appear-and-roundify) {
  0%   { opacity: 0; @include border-radius(2px);  }
  100% { opacity: 1; @include border-radius(10px); }
}

1_output.css

@-moz-keyframes appear-and-roundify { 0% { opacity: 0; -moz-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -moz-border-radius: 10px; border-radius: 10px; } }

@-webkit-keyframes appear-and-roundify { 0% { opacity: 0; -webkit-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -webkit-border-radius: 10px; border-radius: 10px; } }

@-o-keyframes appear-and-roundify { 0% { opacity: 0; -o-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -o-border-radius: 10px; border-radius: 10px; } }

@-ms-keyframes appear-and-roundify { 0% { opacity: 0; -ms-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -ms-border-radius: 10px; border-radius: 10px; } }

@keyframes appear-and-roundify { 0% { opacity: 0; border-radius: 2px; }
  100% { opacity: 1; border-radius: 10px; } }

2_keyframes.scss

@mixin keyframes($name,
                 $moz: $experimental-support-for-mozilla,
                 $webkit: $experimental-support-for-webkit,
                 $o: $experimental-support-for-opera,
                 $ms: $experimental-support-for-microsoft,
                 $khtml: $experimental-support-for-khtml,
                 $official: true)
{
  @if $moz {
    @include with-only-support-for($moz: true) {
      @-moz-keyframes #{$name} { @content; }
    }
  }
  @if $webkit {
    @include with-only-support-for($webkit: true) {
      @-webkit-keyframes #{$name} { @content; }
    }
  }
  @if $o {
    @include with-only-support-for($o: true) {
      @-o-keyframes #{$name} { @content; }
    }
  }
  @if $ms {
    @include with-only-support-for($ms: true) {
      @-ms-keyframes #{$name} { @content; }
    }
  }
  @if $khtml {
    @include with-only-support-for($khtml: true) {
      @-khtml-keyframes #{$name} { @content; }
    }
  }
  @if $official {
    @include with-only-support-for {
      @keyframes #{$name} { @content; }
    }
  }
}

3_experimental.scss

@mixin set-experimental-support($moz: false, $webkit: false, $ms: false, $o: false, $khtml: false) {
  $experimental-support-for-mozilla: $moz;
  $experimental-support-for-webkit: $webkit;
  $experimental-support-for-microsoft: $ms;
  $experimental-support-for-opera: $o;
  $experimental-support-for-khtml: $khtml;
}

@mixin with-only-support-for($moz: false, $webkit: false, $ms: false, $o: false, $khtml: false) {
  // Capture the current state
  $original-moz:   $experimental-support-for-mozilla;
  $original-webkit:    $experimental-support-for-webkit;
  $original-o:     $experimental-support-for-opera;
  $original-ms: $experimental-support-for-microsoft;
  $original-khtml:     $experimental-support-for-khtml;

  @include set-experimental-support($moz, $webkit, $ms, $o, $khtml);

  @content;

  @include set-experimental-support($original-moz, $original-webkit, $original-ms, $original-o, $original-khtml);
}