What happens, when, in the words of the article, you "Do not be constrained by questions of usability, legibility, and flexibility"?
The answer, it appears, is their re-design of HN: https://interface.fh-potsdam.de/future-retro/HN/ which I find unusable.
The expanded view with lines between comments and their parents which scroll independently is literally the worst website UI I have ever seen. And I recall some terrible flash abominations.
That is, create a work of art (as opposed to utility). Okay, art is nice, too.
But mixing artistry and utility is quite hard. Designing a nice, livable house, or a store, is a craft. Building fancy sand castles is a (pastime) art. But mixing the fancy free-form with being actually usable by thousands takes lots of work, resources, and time, because what you'd have to build is a cathedral. (If you think modern technology makes it simple, look at the history of building the Sagrada Familia in Barcelona.)
Web sites have a similar popular anti-feature that's included for reasons of social status: low contrast text. I think this is popular because ability to easily read it signals both health (good eyes) and wealth (good screens).
Perhaps. But I always presumed the good eyes, good screen, and lack of empathy belonged to the designer. These same site too often seem to have experiences based on an ultra-fast connection, as opposed to a wonky 4G (at best) connection.
You might be right. But I've sat in meetings and/or shared office space with low-UX-IQ designer / frontend types.
Fortunately extensions like Stylus allow to tweak page styles to make them more usable.
That should barely be the case with a good layout and hyphenation algorithm (or a competent printer in the old days). What word processing software does is not necessarily the best possible way to produce an even justified layout.
I think you can even go back as far as 1000 years to find those same standards. https://en.wikipedia.org/wiki/Carolingian_minuscule
True, but print media formats got even more simplified in web. The reason? Responsiveness constraints.
Many times I've seen great advanced "article-like" designs for desktop from ambitious designers. It turned out that making them easy to manage in CMS, stable in all possible combinations (or implementing good validators) and at the same time logical and good looking on mobile was extremely hard and not worth it.
This led to further simplifications...
... and then Medium looks like Medium.
There's not much place for creativity on mobile. And this is where most of the "fast content" is read.
For ~500 years, books look approximately the same: normally paged sideways (not top-down), with some margins for handling, with text in rows or columns (depending on the writing system), some chapter structure, and index / contents page, page numbers, covers of more durable material to protect the pages, with some kind of a title on the top cover, etc.
Those features don't just exist because of tradition or technical limitations. They mostly exist because they are convenient, useful, and logical.
But they also exist because people expect them, from times of handwritten books. They put the skills people already had to good use. They created a visual language which is easy to pick up and easy to use, both for readers and typesetters.
Most web sites are a logical continuation of books, magazines, newspapers, etc. No wonder they actively adopt the time-proven, well-working concepts from the print media.
Forms have a much shorter, and much less rich history outside web, and here experimentation was wild; a lot of sites do forms quite differently. Though some common language (like labels, placeholder text, pre-validation, etc) already has formed. OTOH even checkboxes are not yet a commonly accepted visual concept; some e.g. prefer "switches", iOS-style.