CSS is a top-down language, not a constraint language. It is not surprising that you cannot construct things in the same way as you would in a constraint-based layout system.
I am sure I could discover an equal "simple" example that is easy in CSS but difficult/impossible in GTK.
That you cannot use a hammer in the same way as a crowbar, even though they are both blunt instruments, is not a condemnation of the hammer. So yes, you do need to evaluate what the right tool for the job is - like JS to do a quick width-match on the labels' natural widths - after the CSS did most of the rest. That is not "bla-bla-blah," that's just knowing the limits of your tools.
The frustration seems to be the impression that CSS should be able to solve any conceivable layout desire on the web, which isn't its charge: CSS is first and foremost made to style web documents, which by-and-large don't fit as easily into the constraint-based-layout box as a traditional desktop app would. Since this kind of desire often crops up in desktop-like SPAs, where JS use is endemic already, it's not wrong to use a little bit more JS to nudge the visual experience a little closer to what you'd see on a desktop app.
I guess my takeaway is, CSS doesn't promise the world, so if you come in expecting that, you'll walk away disappointed. Luckily there are many tools in the web dev toolkit to achieve your engineering desires, and it's not verboten to use them (aside from a philosophical desire to avoid JS, but if the biggest problem for your page when JS is disabled is that the labels are slightly different sizes, I think you're doing a great job already).
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".
It was my bla-blah, so please read it as etc-etc, and if you feel offended by that and by my tone, I apologize. But it would be fair if you also excused my huge disappointment after following these exact ideas, spending days and finding out that they do not work or are completely non-viable in vivo. If any css loving guy implemented the puzzle above (and a half of other common issues) as a library that could be simply added to a job-grade project, they are welcome to share it (or sell, such an easy $$$). I've been in these discussions and done what was suggested, that's why I listed all these reasons word by word, before anyone had a chance to repeat them, and that's where my criticism comes from. I just cannot sit here and silently watch how they praise a thing that is not remotely as good as is claimed. How many man-centuries have to be spent until we all admit the undeniable?
I am sure I could discover an equal "simple" example that is easy in CSS but difficult/impossible in GTK.
That you cannot use a hammer in the same way as a crowbar, even though they are both blunt instruments, is not a condemnation of the hammer. So yes, you do need to evaluate what the right tool for the job is - like JS to do a quick width-match on the labels' natural widths - after the CSS did most of the rest. That is not "bla-bla-blah," that's just knowing the limits of your tools.
The frustration seems to be the impression that CSS should be able to solve any conceivable layout desire on the web, which isn't its charge: CSS is first and foremost made to style web documents, which by-and-large don't fit as easily into the constraint-based-layout box as a traditional desktop app would. Since this kind of desire often crops up in desktop-like SPAs, where JS use is endemic already, it's not wrong to use a little bit more JS to nudge the visual experience a little closer to what you'd see on a desktop app.
I guess my takeaway is, CSS doesn't promise the world, so if you come in expecting that, you'll walk away disappointed. Luckily there are many tools in the web dev toolkit to achieve your engineering desires, and it's not verboten to use them (aside from a philosophical desire to avoid JS, but if the biggest problem for your page when JS is disabled is that the labels are slightly different sizes, I think you're doing a great job already).