While the source looks like JSON, the page is still sent by the server as text/html. Browsers are so resilient to malformed markup that actually parse the content inside the <html> tag at the end of the document (firing the JS script that render the actual page), ignoring all the (pseudo) JSON around.
The page is rendered in QUIRKSMODE because the source is missing the <!DOCTYPE html> at the beginning of the document, to resemble valid JSON.
Well, at least that's clever!
