display an (x)html document structure in a new window

For debugging purpose, I need to create an new xml document popup to display the (x)html source structure of my current document.

But the following code does not work:

var w = window.open();
w.document.open('text/xml');
w.document.write(window.document.documentElement.innerHTML);
w.document.close();

It seems that document.open() does not accept contentType anymore.

Is there any other solution ?

Just put a textarea in your existing page and copy the innerHTML into the textarea.