OT: can someone here tell me why there's a continue reading button after a single paragraph that just repeats the heading? I've seen this trend on other sites and it's incredibly frustrating, especially if it forces a page reload

I would think it's a measure of "user engagement" in a way - how many users care to actually click to read more vs. just skim the first little bit and click away.

So the footer becomes more accessible in case of lengthy pages. In case of news websites, it's to better surface ads, in this case it's to surface recommendations

The app they are building is a Markdown editor that can evaluate code snippets live by sending them to a REPL. This functionality was inspired by Emacs org-mode, they go into some details in the first few minutes of the video.