>No filesystem

It's limited but there is filesystem access and you could always just make your own ramfs.

>No network

Websockets are allowed to connect to arbitrary IPs. It would be nice if you could give apps permissions for TCP/UDP but what most people do is have a websocket "adapter" server of some kind that at least lets you connect to the local services. Others (like jslinux) will do TUN/TAP over websockets which is horrendous but does technically work.