Or is it proof that CSS has "jumped the shark", adding in a plethora of features that have no place in a "stylesheet"?
I guess I am asking, if modern CSS is so awesome, it's awesome compared to what exactly?
Duct tape is awesome for the same reason -- even though there are several effective use cases for duct tape where a different tool would technically be "better" for the job.
No doubt the very loudly opinionated not-web-dev community, who have So Much To Say about web devs and web tech, have produced an uncontroversial, perfect layout system, styling system, and language to produce the replacement for this awful web tech. Where is it? What is it? Please provide (the better version of) a hyperlink to the docs.
So yes I will argue that Qt6 and GTK are better than flexboxes.
Because you asked for it, here's some documentation to a UI framework which I believe to work better than the web stack: https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/index.html
You can keep it high level but your comment makes me think you have something in mind, and I'm honestly curious.
In CSS on the web, just centering a div has historically been a problem. We have flexbox now, but what if CSS was designed with our current needs from the get-go?