The reason Javascript won't do the trick is that in order to send the correct Javascript, the server needs to have all the information necessary to decrypt all

The reason Javascript won't do the trick is that in order to send the correct Javascript, the server needs to have all the information necessary to decrypt all the content anyway. So there's no point in the encryption at all, because the information will pass through the server unencrypted at some stage and can be captured. If this were implemented as a browser feature or plugin, machines with the user's key information installed would be the only devices capable of decrypting the content sent by the server.