That's Reveal.js / Slides.com format. It became very popular in 2010s. The idea behind the 2-d navigation is that you can use left-to-right to move between chapters, and move down to dive into a specific chapter. This allows you to skip chapters due to time constraints. Or hide gnarly details about something so that these specific slides do not break the flow of presentation but still having them available for the audience online. Or, having slides announcing demos, but if demos do not work the down slide would have a video demonstrating how the demo is supposed to work. Many possibilities like this. Also the slides are produces using Markdown, so the format was appealing to many authors.

However, doing chapters well turned out to be tricky. Ideally you want them to be of similar size and have 3 to 7 of them in the talk, but many presentations aren't structured like this. The rise of Slideshare and SpeakerDeck for sharing slides in mid 2010s caused this 2-d navigation to go out of favor: those services only support linear static slides. This is also a reason why people use fewer animations in slides nowadays and why tools like Prezi didn't catch on (that was another presentation tool with non-standard navigation that went out of favor very quickly).

Many people still use Reveal.js to make their slides but they stick to left-to-right nav only.

Going to the link and just hitting the spacebar worked for me. Next slide, and so on. Firefox/Linux.

I have the same problem with the mouse (little page marker overlay covers the down arrow).

But using keyboard arrow keys work for me.

Standing in that hole without shoring... no thanks. Impressive project nonetheless though

Ugh, deep links should be part of the path, and anchor should be where on the page to scroll. Very annoying slide software. If the content weren't so good I simply wouldn't bother.

It's a kind of crappy slide deck, not a proper home page. Even worse, the link drops you into the middle of the deck. (TBF, it wouldn't be so bad if you know that it's a slide deck when you load the page.)

Try using the arrow keys to navigate. It took me multiple tries to get it figured out.

Use up/down to navigate within a chapter/topic. Use left/right to switch between topics.

Very good, actually. But you have to nudge them slightly. Tell them you prefer the modern version of the language, with gradual typing† and function signatures, and you'll get very good results. Perl interpreter comes standard on modern OSes and due to permissive licensing and impeccable backwards compatibility you can always assume you deal with very modern versions of Perl.

I write Perl scripts that are 10-100 lines of code, and at this size Perl is a Strictly Better Bash: better syntax, some type checking, better text support, and still effortless calls to external processes: essentially you put a command with arguments in backticks, and you get it's output. Ruby can do it too, but not all systems have it. Python is another obvious choice but calling external commands in it is annoying. I also use Perl for some one-liners as a better `sed` for text replacements.

† Perl nowadays have TypeScript-style type checking for function parameters. So, while the syntax is wild sometimes, the language is much better than it used to be.

Codex for some reason sometimes runs Perl instead of Python to work with local files