I have read the following article. https://inspiringingenuity.net/2015/12/03/htmlsdk/. I have pulled apart several plugins in that are already installed on my machine. But I am about at the end of what I can self discover.
Where can I find actual, in-depth documentation for building HTML/Javascript SDK plugins?
The HTML GUI SDK is available as a beta in 11.5. You can find documentation for both the HTML GUI SDK (beta) and the Python SDK (alpha) online. We'd love for you to provide your feedback here.