Book (epub) do not show sub-entries in the table of contents

[carif]

Describe The Bug
A book in the ePub format can have a hierarchical structure of chapters with sub-chapters etc. Those sub-chapters do not appear in the table of contents view in Jellyfin. Only the top-level chapter's get displayed.
In the screenshot section are some screenshots of an example file to illustrate the problem (Jellyfin vs Calibre). But really, any chapter names and chapter-structure has those problems. I could've move some chapters to the top level and added sub-chapters to them, too, e.g.:

Cover
Made in Abyss 08
- Chapter 01
- Chapter 02
Side Story
- Chapter 01
- Chapter 02

Steps To Reproduce

1. Go to your book library.
2. Click on a book to open it. It has to be an ePub file with sub-chapters!
3. Click on the button for the table of contents.
4. See error

Expected Behavior
Show a list of all chapters.

Screenshots
The problem:

How it should look like (image from calibre):

System (please complete the following information):

• Platform: Fedora Linux
• Jellyfin + Jellyfin Web Version: 10.8.9

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[carif]

This is still an issue in 10.8.10.

[vincent]

How the nested chapters should be displayed ?

I saw some readers flatten the list, making all chapters to appear on top-level.
If they are shown as a tree, should we fear about stupidly-deep nesting ?

Also, atm the font size is recursively bigger, caused by a font-size: 120%

[vincent]

Here is a first proposal, open for any feedback.

[carif]

Here is a first proposal, open for any feedback.

I would probably make the "Chapters" and "Part One" things bold to make it a bit easier on the eyes. I'm not sure what other eBook Readers do here, but Apple makes them bold and the Sections and Sub-sections normal and it looks really nice.

I saw some readers flatten the list, making all chapters to appear on top-level.
If they are shown as a tree, should we fear about stupidly-deep nesting ?

Flattening would be an option, but it would make it harder to understand which sub-chapter relates to which chapter. Regarding the question whether we should fear about studiedly-deep nesting. In theory yes, in practice, all the books I've ever seen go at max to the sub-section depth of your first proposal.