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).