<h1>Original Title</h1>
<p>Original Paragraph</p>

html, body {
  display: grid;
  height: 100%;
  align-content: center;
  justify-items: center;
}

const simpleHTML = '<h1>New Title</h1>'
const body = document.querySelector('body')
const firstChild = body.querySelector(':first-child')

// insert the new element as after the first child
firstChild.insertAdjacentHTML('afterend', simpleHTML)