We aren’t going to make Observable accessible under HTTP - it’s intentionally built as a secure site, and very soon all HTTP sites will be marked as not secure by Chrome.
We do plan to implement some nice features for this sort of usage within an intranet that will preserve our security promises, but they’re all a little further down the road.
For now - most likely, your browser isn’t trusting the self-signed SSL certificate. You might want to try visiting the page / resource in question directly (not just loading it through Observable) and seeing if your browser gives you an option to trust the certificate and load the page. After you do that, your browser should be able to trust that self-signed certificate and load the resources.