I am having a problem with that preview tab in network section. When HTML is returned I am seeing the raw HTML in both preview and response. Sometimes, but very rarely, I HTML is rendered properly in Preview tab.
Am I doing something wrong or it's just some Chrome bug?
Thanks
I was testing this as well. What did it for me was adding the http header.
header('HTTP/1.1 500 Internal Server Error');
Then of course some proper markup formatting. But the status code was what I needed to convince chrome to render the preview.