table of contents missing

[jlduan]

Hi,

Thanks for the very useful project. I usually use RMarkdown to covert my R code into html files. Recently I noticed that the table of contents are missing when rendering with htmlpreview.

Here is the example:
https://htmlpreview.github.io/?https://github.com/jlduan/Replica/blob/master/s41421-021-00316-8/notebooks/analyze.html

It used to work:
https://htmlpreview.github.io/?https://github.com/jlduan/Replica/blob/master/j.stem.2021.04.031/notebooks/analyze.html

Thanks!