AMMDI is an open-notebook hypertext writing experiment, authored by Mike Travers aka mtraven. It's a work in progress and some parts are more polished than others. Comments welcome! More.
First, the CSS required is nuts. But you don't have to worry about that, unless it breaks or looks ugly in your world.
Sidenote are specified with a special syntax that I probably can't reliably quote here, hm, let's try it:
{{sidenote Whatever fancy stuff you like, including links [[nebulosity]]}}
An older version of goddinpotty used a different way of specifying sidenotes, using a separate block and block-reference, but that proved unwieldy and the feature has been removed.