This first command will create a new body element.
document.body = document.createElement('body');
We still have the head element with any styling that is present. You can see this by putting in some simple markup:
document.body.innerHTML = '<h1>A heading</h1><div class="BLOCKone">Block</div>';
You will probably see that the heading is styled in the way that the website you are looking at styles headings.
We can clear that out using:
document.head.innerHTML = ''
One is still left with any Javascript code which is present. Haven’t thought of an easy way to clear that yet.
Here’s the lot in one easy code fragment to copy paste into the Javascript console:
document.body = document.createElement('body');
document.body.innerHTML = '<h1>A heading</h1><div class="BLOCKone">Block</div>';
document.head.innerHTML = ''