src/language-css/parser-postcss.js/stringifyGroup
function stringifyGroup(node) {
  if (node.group) {
    return stringifyGroup(node.group);
  }

  if (node.groups) {
    return node.groups.reduce((previousValue, currentValue, index) => {
      return (
        previousValue +
        stringifyGroup(currentValue) +
        (currentValue.type === "comma_group" && index !== node.groups.length - 1
          ? ","
          : "")
      );
    }, "");
  }

  const before = node.raws && node.raws.before ? node.raws.before : "";
  const value = node.value ? node.value : "";
  const unit = node.unit ? node.unit : "";
  const after = node.raws && node.raws.after ? node.raws.after : "";

  return before + value + unit + after;
}