Can a website prevent an element from being added to the already loaded page?

If the code below is run in DevTools or in the browser’s address bar, the h1 element will be inserted at the end of a website’s page.

const elm = document.createElement('h1')
const txt = document.createTextNode('New text')
elm.appendChild(txt)
document.body.appendChild(elm)

Can any site prevent this from happening using, for example, MutationObserver?