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test.scss

@mixin optimize-font-size($sizes) {
  @each $r in $sizes {
    $w: $safe-for-full * $r * $r;

    @media (min-width: $w) {
      font-size: percentage($r);
    }
  }
}

html {
  font-size: 100%;
  @include optimize-font-size((1.125, 1.3125, 1.5, 1.6875, 1.875));
}

test2.scss

@mixin adjust-font-size($sizes) {
  @each $s in $sizes {
    $r: $s / 16px;
    $f: min-width;
    $v: 58.24969em * $r * 1.25;

    @if ($r < 1) {
      $f: max-width;
      $v: $safe-for-content * $r * 0.75;
    }

    @media ($f: $v) {
      font-size: percentage($r);
    }
  }
}

html {
  font-size: 100%;
  -webkit-text-size-adjust: 100%;
  -ms-text-size-adjust: 100%;

  @include adjust-font-size((14px, 18px, 21px, 24px, 27px, 30px));
}

レスポンシブフォｗｗｗ