There are two alternative behaviors I can think of: 1. The service operator prerenders link previews on its own servers, then sends them to the client, which m

There are two alternative behaviors I can think of: 1. The service operator prerenders link previews on its own servers, then sends them to the client, which means that it needs to be privy to the contents of messages. I suspect this is already the case for Skype, but it's a feature that (say) Signal can't do this. 2. The message sender prerenders the link preview. Where link previews are interactive (I'm always impressed by linking Bandcamp in Slack, for instance), one could easily accomplish the same with some malicious JavaScript. I'm not sure any particular behavior is "better".