function toCssText (rules, callback) {
  if (!rules) {
    return '';
  }
  if (typeof rules === 'string') {
    rules = parse(rules);
  }
  if (callback) {
    forEachRule(rules, callback);
  }
  return stringify(rules, nativeCssVariables);
}
@param {string|StyleNode} rules
@param {function(StyleNode)=} callback
@return {string}