For me it's the horrible layout. For God's sake, stop making narrow columns of text. Having the text take up most of my monitor is much more pleasant to read.

I think the opposite. When text in a webpage takes up all my monitor's width, I go into Developer Tools and manually add a max-width rule so that I can read the text comfortably.

And AFAIK, all HCI literature seems to agree with me.

Long lines of text cause significantly more eye strain than reasonably short ones. Generally, one should try to have ~80 characters per line of text.

Research suggests optimal line length is 50-75 characters

https://baymard.com/blog/line-length-readability

Discretional ligatures… well, they require discretion, which the author seems to sorely lack.

Haha, I used iPhone’s reader mode, which I do most of the times.

The "t" or the "ct" ligature?

Wow I am reading it at 170% zoom, and in the fourth paragraph the word "distribution" which contains the "st" ligature is automatically cut and "hyphenated" between the "s" and the "t" letters. But the ligature remains : half the ligature at the end of one line, and the other half of that ligature at the beginning of the next line ! This looks wrong. CSS has probably missed an edge case here. Or is it the job of some "text renderer" in the browser ?

"st", also.

It's the "historical-ligatures" feature of used font, if you aren't in the reader mode already, F12 and

    document.body.insertAdjacentHTML('beforeend','<style>p { font-variant-ligatures: common-ligatures discretionary-ligatures contextual;}</style>')

should turn it off. (Was "too much" for me either.)

But besides this, I found typography of that article quite nice; interesting that there are thin spaces before "?" and "!" and wide spaces (not double spaces) after sentences - also "old school" (and often frowned upon). I guess some WP plugin does it, but I admit don't remember seeing seen this anywhere else recently. (And I like it.)