There's decades of UI frameworks where the structure of the elements/widgets dictates the layout. HTML/CSS doesn't do that, because it is clearly made to style text documents, not UIs.

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