[GeoffIsTheBest]

This is a cool app. I can't seem to find any more info about it. Does it have an API? Browser plugin? Open source?

[tsukurimashou]

You can see some info on their frontpage https://outline.com/

There are some links at the bottom.

As far as I can tell it seems proprietary software, couldn't find the source or anything about it being opensource anywhere.