Firefox Reader Mode

I am trying to figure out how to write a website which will support Firefox Reader Mode. I like Reader Mode because it is a nice way to remove the frills from a website to just read the content of a specific article.

I have been fiddling with this for quite a bit now and I found some information online that suggests that it is a heuristic. I find it somewhat troubling that I cannot figure out how to trigger this heuristic. After expanding this page a bit, it seems the magical trigger is 4 <p> tags.

I am going to create a bugreport and ask for some documentation. Done.

Here is an example of a website where it works: https://jrl.ninja/etc/1/. I was looking at it because I also need some nice css but that is a different problem.