It would be nice if browser offer per-origin encryption of files so that if an exploit at a future date allows x.y.com to access the files of y.z.com they would

It would be nice if browser offer per-origin encryption of files so that if an exploit at a future date allows x.y.com to access the files of y.z.com they would only be getting encrypted blobs (unless the encryption/decryption API is also exploited, but now we're talking about multiple exploits rather than just one).