document.documentElement.innerHTML = "whatever";
I think a browser rightfully assumes a page with content-type
text/html will always be a web page - so whilst you may do something like...
document.body.innerHTML = '';
It will still have some HTML hanging around.
You could try...
document.documentElement.innerHTML = '';
...which left me with
Yi Jiang did suggest something clever.
window.location = 'about:blank';
This will take you to a blank page - an internal mechanism provided by most browsers I believe.
I think however the best solution is to use
document.open() which will clear the screen.