With “nesting of headings” I meant that the outline for a level 3 heading must be a child of the outline of the preceding level 2 heading. Like that:
On the left side, “Delving deeper into CSS” is marked as a top-level heading. On the right side, it’s expanded to show the three 2nd-level headings below it. I’ll clean up and comment the code, and then I’ll post it in a new thread, namely here.