I definitely prefer this to the "beautiful" pages that try to capture the feel of old print magazines.
Conversely, for reading Markdown, I prefer a rendered version in a proportional font.
So both have better "readability" in different ways. Faster/easier or more accurate. Which implies to me at least that mono is better for everything but body text even when things are ideal. I don't think it is possible to have a single proportional font that works for both definitions of the word "readability".
On modern high density displays serif fonts can also work fine. But not all displays out there are actually of that high density.
That's just one aspect though. There has been a lot of research over the years which for some reason is often ignored.
I was about to write out a lot more and while looking up various sources stumbled upon a medium post that dives into things quite deeply: https://medium.com/@pvermaer/down-the-font-legibility-rabbit...