You just switched from blaming me for not knowing css to blaming me for not using the right tool, as if I had a clear way to do that. It is nothing new and was propheted in bla-blah paragraph. But if you ever tried to size-match in JS, you know that it is not a trivial task. First, an element must be temporarily placed into document.body for all styles to apply correctly. Second, it must repeat all of the hierarchy it was in, which requires to break/duplicate a component encapsulation in your code. Third, in a narrow-width situation you simply cannot guess what surrounding containers would do to the effective width/height based on an intrinsic geometry. Not even speaking of cpu/battery issues, these three alone make it possible only in theory. And no, there is no library for that to "just work" in your project.
I am sure I could discover an equal "simple" example that is easy in CSS but difficult/impossible in GTK.
I doubt it, really. I mean, you could find something for base gtk (which is already out of reach compared to css in ui geometry, because it was done for it), or for the fact that gtk is not a networking engine, but it is just a bunch of predefined containers, calculation phases and signals in C. It is easily extensible in a native way and hides no knowledge about how everything looks or works. Gtk is an example of how you expose distinct particles, pre-implement few useful combinations, and then users build a universe with them. Css is an example of ritualistic black box that hides almost everything about its geometry and requires a non-trivial effort for really basic things. E.g. in css, you have to struggle with dumbest things like a one-pixel vertical scroll in a single-line input. My complaint is not "css is not gtk-like", it is "css is incomplete and unreasoned AF in its own model".