JSFiddle - React, Tailwind, and code Playground

HTML

<mark class=obj><u><a>т</a><strike><b>уп</b><a>и</a></strike><a>к</a></u></mark>

<div id=w></div>

JavaScript

var obj = document.querySelector('.obj')

getChildren(obj)

function getChildren(obj, collection = []) {
  let children = [...obj.childNodes]
  return collection.concat(
    children,
    children.reduce(
      (collection, child) => collection.concat(getChildren(child)), collection
    )
  )
}

console.log(getChildren(obj))

w.textContent = obj.textContent