Javascript - Window open use CDN

Question

Simple question. I cannot get a new opened window to use CSS.

var previewWindow = window.open();
$(previewWindow.document.body).append(data);

The data is just a stringbuilder generating html from our backend.

The problem is, I want the newly opened tab/window to use Twitter bootstrap, either CDN or use the file.

I cannot get it to work with, what I tought should be the solution

$(previewWindow.document.head).append('cdn or path goes here')

I cannot seem to find anyone else with this problem, or I need a lecture in Google.

As you can see, in the head the CDN is properly added. But the html isn't using any of the CSS.