It is not the determinism that’s a concern. JavaScript can test the capabilities of your machine and then create a new connection to transmit its results back t

It is not the determinism that’s a concern. JavaScript can test the capabilities of your machine and then create a new connection to transmit its results back to another computer. Even in the hypothetical scenario when you are only allowed to download messages and not upload messages, things are hard: because you have a fully programmable language at your disposal, you can create a system to transmit information by a sequence of reads with JavaScript. So the only safe way is to pre-download everything, then turn off the network and only then evaluate JavaScript in an environment without network or storage. FSF is not against such a mode of operation, but it becomes cumbersome for most people. And as the development of browsers consolidates, the fraction of the web that can be used in that way (for those who want to be super careful) becomes smaller.