A really useful feature, which I hope gets broad browser support: Subresource integrity.
It basically allows a reference to include the SHA256 hash of the expected result. As such, you could link to a download on another site and verify that the downloaded file was not tampered with.
Another use I would like is if the browser can verify that the downloaded file is not corrupted.
Thanks to CodesInChaos for pointing this out!
— CodesInChaos (@CodesInChaos) January 16, 2015