Block non .onion content on .onion addresses (mixed content blocking)

The .onion URL for a given THS instance is a fingerprint of the public key, thus ensuring authenticity of the service. For this reason, some assume the same security assurances for .onion addresses as they would for https, with the added assurances that hidden services provide. For instance, the major browsers have chosen to not load http resources when accessing an https site, blocking mixed content. However, there is no protection against mixed content being loaded in the TBB for .onion addresses when they include resources from http URLs. For any .onion URL which includes http resources, an attacker controlling an exit node could perform a Man in the Middle attack, providing malicious javascript which modifies the content of the DOM.

One would hope that an http THS would never include remote resources from an http site if they would like to protect their users. In fact, one would hope that a THS would never load any resources at all from a source they do not control. But this is no guarantee that they won't. It seems like a good security measure to disallow http resources from being loaded in TBB.