Note that we can enter some sample text for a simple preview the element/widget, and we can also enter some more complex wikitext for a more in-depth preview.
Here, I've created a tiddler and I'm transcluding it in the complex preview, so that I can see how the details element (based on cDeets) looks with the summary element (based on cDeets_S).
That complex sample shows in the editor for both of those classes.
Note the "information" buttons here and there. Learn about a thing by clicking right there where that thing is.