src/language-html/ast.js/Node/walk
class Node:  walk(fn) {
    for (const NODES_KEY in NODES_KEYS) {
      const nodes = this[NODES_KEY];
      if (nodes) {
        for (let i = 0; i < nodes.length; i++) {
          nodes[i].walk(fn);
        }
      }
    }
    fn(this);
  }