`[unknown div]` Warnings in pull requests

I'm not sure what causes this and I was told by @zkamvar that this is harmless, but putting it here in case others have the same question.

When looking at pull requests that have been reviewed by Github actions, there are many warnings related to `div`s, which look like

```
Check warning on line 340 in episodes/01-run-quit.md
GitHub Actions / Build markdown source files if valid

[unknown div] row
```

See for example [here](https://github.com/swcarpentry/python-novice-gapminder/pull/644/files).