Every now and then it is claimed that plasma can also run osx widgets, however the html-based one, how can they be easily identified?

has anyone ever compiled a list of the working osx widgets on plasma?

which osx widgets did the developers test before claiming plasma-compatibility?

at the moment I could not find a single osx widget working on plasma...could anyone post his positive results if he / she had any?

It works only for HTML widgets. There was some work on reverse-engineering (due to Apple's terms) the Javascript ones, but as far as I know, the effort has stalled.

Which specific widget would be an "html" one?
I tried one million of them and no one worked or was even installed.

In fact, if I try to use the plasmapkg command to install the apple widgets I always get the "installation failed" response.

I am not sure as I don't use them. You may want to open a bug on bugs.kde.org (carefully describing the steps to reproduce the problem) in this case (failed installation).