function walkStyleRules(node, iterator) {
  node.nodes = node.nodes.filter((rule) => {
    if (hasNestedRules(rule)) {
      walkStyleRules(rule, iterator);
    }
    rule._other = undefined;
    rule.filterSelectors = filterSelectors;
    return iterator(rule) !== false;
  });
}
Recursively walk all rules in a stylesheet.
@private
@param {css.Rule} node       A Stylesheet or Rule to descend into.
@param {Function} iterator   Invoked on each node in the tree. Return `false` to remove that node.