Простенькая имитация ДОМ дерева

DOM.js

let html = document.children[0];

for (let i = 0; i < html.children.length; i++) {
  let child = html.children[i];
  console.log(child.tagName.toLowerCase());

  for (let j = 0; j < child.children.length; j++) {
    let innerChild = child.children[j];
    console.log('|---' + innerChild.tagName.toLowerCase());
  }
}