A couple of the books I've harvested recently use definition lists for some of their terms. LibreTexts formats anything with the <dl> tag so it starts out collapsed, and toggles if you click it. It doesn't look right on some of the pages.
For example, on this page:
The sections for "targets:" and "repeated_nodes:" start out collapsed.
You have to click them to get the description.
Usually collapsed definition lists are also blue like a link as a hint to click them, but the <code> format for these terms makes them gray, so it's kind of easy to miss.