docs: function cssToHead (css, id, once) {
    if (once === undefined) once = true;
    if (once) {
      if (___inserted.indexOf(id) === -1) {
        injectCss(css, id);
        ___inserted.push(id);
      }
    } else {
      injectCss(css, id);
      ___inserted.push(id);
    }
  }