I was really excited to see GSS (http://gss.github.io), however at the time it was far too slow to be usable in real projects.
I imagine container queries will totally change the way I write CSS as soon they are broadly supported.
Of course this would not make for a truly constrained-based layout definition. But it would at least make working this way somewhat possible.
The example where flex-directon is switched using a container query is spot-on. That's exactly ly what I sometimes miss being able to do in HTML+CSS.
Apparently there is a JS polyfill (awesome given the problem) but I haven't tried it and I assume the performance implications are bad.
I was really excited to see GSS (http://gss.github.io), however at the time it was far too slow to be usable in real projects.