Material Design sp units (px unit that scale to the user's font settings) using rems

gist.css

html {
  font-size: 14px;
}

/**
 * PostCSS
 * Use a custom PostCSS to convert #sp to rems by dividing the number by 14 and expressing it as #rem
 */

body {
  /* Device  */ font-size: 14sp; /* -> 1rem */
  /* Desktop */ font-size: 13sp; /* -> 0.92857rem */
}

/** SASS */
@function sp($sp) {
  @return ($sp / 14) * 1rem;
}

body {
  /* Device  */ font-size: sp(14);
  /* Desktop */ font-size: sp(13);
}