I like your proposed solution, @jodamo5.

An alternative I'd like to see is having the list collapse by default as Wikipedia does.

@richtabor, @jasmussen, I would appreciate any recommendation here from a design perspective.

This seems like a valid option to build into the inspector, where there's plenty of room.

I would also tend to agree that the dropdown proposed, defaulting to showing all heading levels, would likely be the best simplest interface; this is presuming we will always want to show H1s if available. We definitely want to avoid a checkbox system where you can show only H2s and H4s, for example.

One question would be whether it's a toggle, on by default, with the label "Show all heading levels", and when untoggled it surfaces the dropdown below. Or whether the dropdown can stand on its own.

The other is what labels to show, "Lowest level" isn't that clear to me. To get the conversation started, perhaps:

Heading levels
[Show all heading levels ▾]
Select which heading levels to show.

— with dropdown options:

Show all heading levels *
Headings 1 and 2
Headings 1 to 3
Headings 1 to 4
Headings 1 to 5
Headings 1 to 6 

• default.

Mainly it would be good to avoid the HTML markup "H1", "H2" terminology, as that likely won't resonate with a majority of people.

Thank you, Joen!