This is a very small add-on I've developed as a proof-of-concept to synchronize video playbacks in almost every website with another browser with P2P connections, without needing a server. With this extension you can watch videos online with your friends without explicitly needing to tell each other the duration or actions.

It's all free and open-source, so feel free to check it out and leave a comment :)

Source code: https://github.com/kaangiray26/sync-everything