[tex4ht] Subsubsections do not show up in TOC

[Michal Hoftich]

> Okay, thanks for the info - glad to know that it isn't just me! In the end, I decided to edit the issues manually with the Calibre Editor so that I could hit "Publish" at Amazon KDP today. But I'll make sure to test my build more regularly in the future - there is always a next book, after all.   ;)

Ah, that's another possibility. Anyway, I found some issues in the
tocdepth warning thanks to your post, which didn't cause issues in
your use case, but would produce a wrong result in \tableofcontents.

I also got another bug report for TeX4ebook, so maybe I will make a
new release before TL 2021 is frozen.

Best regards,
Michal