Css rule for setting the "Native Font stack" or "system font".

native-font-stack.css

body {
  font-family:
    /* Safari for OS X and iOS (San Francisco) */
    -apple-system,
    /* Chrome >= 56 for OS X (San Francisco), Windows, Linux and Android */
    system-ui,
    /* Chrome < 56 for OS X (San Francisco) */
    BlinkMacSystemFont,
    /* Windows */
    "Segoe UI",
    /* Android */
    "Roboto",
    /* Basic web fallback */
    "Helvetica Neue", Arial, sans-serif;
}

/* based on ideas from https://www.smashingmagazine.com/2015/11/using-system-ui-fonts-practical-guide/ */

/* another approach worth checking out which uses @font-face declaration https://github.com/jonathantneal/system-font-css/blob/f40d7260df72a27e5c917604727dfee048764900/system-font.css
 * so you could just use `font-family: 'system';` anywhere
 */