It means, that you don't need an internet connection for this to work :) (so it is no rabbitmq or so which runs on a server and the browser is just the client)
You can try on the demopage when you
1. play the songs each (for them to buffer a little audio snippet)
2. open the page in a second tab
3. Disconnect from the internt
Still works :D
I'm just asking random questions now and then to get an idea of how things work in the web world. Especially those that aren't mentioned in tutorials.
Which is a good thing :) I am all in for asking questions. If someone can't answer them, it usually means they have not thought about it and are either ignorant or will learn something themselves :D
(like me for your question, as I never tried TabSub with internet disconnected. Was delighted to see it works nevertheless)
But but, you used "offline". I always thought that's the opposite of "online". Looks like it means something else in Newspeak :)