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 <html></html>.
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.