Will extensions only work if the streamer has them enabled.
It would be cool if you could make an extension that viewers can use for any twitch channel that can be used with other viewers that also have the extension.
As I understand it, they will have to be enabled by a Twitch streamer to show up on a particular stream page.
They advertised them to be very open though, so I guess that user interaction in the extension could be possible. Will have to wait for the docs.
“Extensions installed by a broadcaster are automatically visible to any viewer who goes to the broadcaster’s channel page using a desktop Web browser. Viewers can see details about the extension and report an extension for bad behavior.”