code: function clean(root) {
    var test = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 'critical-selector';
  
    var clone = root.clone();
    if (clone.type === 'decl') {
      clone.remove();
    } else {
      clone.walkDecls(test, function (decl) {
        decl.remove();
      });
    }
    return clone;
  }
docs: Clean a root node of a declaration.
  
  @param {Object} root PostCSS root node.
  @param {string} test Declaration string. Default  `critical-selector`
  @return {Object} clone Cloned, cleaned root node.