_demo.scss

@import "_font.scss";

html {
  font-size: percentage($_FONT_SIZE_ROOT / 16px);
}

.foo {
  @include font(23px, 30px, bold);
}

// output:

html {
  font-size: 81.25%;
}

.foo {
  font-size: 23px;
  font-size: 1.76923077rem;
  line-height: 1.30434783em;
  font-weight: bold;
}

_font.scss

$_FONT_SIZE_ROOT: 13px;

@function rem($px) {
    @return ($px / $_FONT_SIZE_ROOT) * 1rem;
}

@function em($em) {
    @return $em * 1em;
}

@mixin font($fontSize, $lineHeight: null, $fontWeight: null, $fontStyle: null, $fontFamily: null) {
    @if ($fontFamily) {
        font-family: $fontFamily;
    }
    font-size: $fontSize;
    font-size: rem($fontSize);
    @if ($lineHeight) {
        line-height: em($lineHeight / $fontSize);
    }
    @if ($fontWeight) {
        font-weight: $fontWeight;
    }
    @if ($fontStyle) {
        font-style: $fontStyle;
    }
}