Content Headings Tree Plugin creates a tree list of the page content headings.
In the Widgets page you will see the "Content Headings Tree" widget.
The shortcode can be used as shown: [headings title="My Headings" size="30" level="3"]
The content headings tree container tree has the built in class "content-headings-tree", so you can use it in your CSS. For the widget, the built in class is "content-headings-tree-widget".
All h headings (i.e. h1, h2, h3, etc.)