This article is an excellent summary of recent trends and philosophies of CSS. The only new, noteworthy styling solution missed (from what I can tell) is StyleX from Meta.
As someone working in this space, it's a huge relief to move away from piles of CSS/SCSS files towards colocating styles with markup.
Maybe I'm the minority here, but with something like Vite I can import a css file directly into a given component's file and call it a day. Haven't worried about scoping in forever, and only need to manage a number of distinct, small, and comprehensible css files.
But they're still separate files. You still have your markup in one place and your CSS in another place, and you need to solve the hardest problem in computer science (naming things) any time you make substantial changes. I think Tailwind has some shortcomings, but I really like that styling and markup live together.
When talking about CSS-in-JS I am reminded of the short lived escapade at Netscape around 1997/1998 when they launched JSSS, only be be shut out by Microsoft ignoring support for it in the increasingly dominant Internet Explorer as they favored CSS. As a result it died and disappeared.
I remember JSSS, but a more vivid memory for me is LiveConnect: another Netscape-only project that was incredibly powerful and groundbreaking: it allowed Java and JavaScript code to communicate directly, e.g. plain JS could invoke the JDK and instantiate Java objects. In retrospect, also a truly terrible idea, but at the time, it was amazing.
As a long-time MUI user myself, I had the same "revelation" about Tailwind recently. Before MUI, I worked at a couple startups where we came up with our CSS franken-framework with homegrown utility classes.
It's just easier to do fast proto-typing with various Tailwind classes, and then refactor common sets of HTML tags with tailwind classes, they become reusable React (or Svelte) components.
TL;DR the author settled on Tailwind. I prefer Tachyons myself (less overhead). I agree that functional/atomic CSS is the way to go.
Separation Of Concerns (HTML <-> CSS) is a LIE. Naming is one of the hardest things in computer science, and folks in the "Separation Of Concerns" camp repeatedly subject themselves to this torture by forcing themselves to come up with "semantic" names instead of just writing the damn styles and seeing which styles get repeatedly used together so that they can be condensed into a class.
As someone working in this space, it's a huge relief to move away from piles of CSS/SCSS files towards colocating styles with markup.