Why doesnt document.body work?

Question

So upon playing around i noticed that although document.body is null, if i instead use documentElement my code works....why?
https://code.sololearn.com/WgGffE44Pach/?ref=app

Answer 1

You just did a simple mistake:

Wrong :

window.onload = start();

Correct :

window.onload = start;

If you do this, both document.documentElement and document.body will work :))

Answer 2

😶