Anyone who has worked with CSS long enough knows that:
1) Rule priority can get out-of-hand very quickly. 2) It's a nice thought to have more powerful selectors, but, taking #1 into consideration, perhaps limiting its power isn't such a bad idea after all.
I was already on-board with using PostCSS modules, https://github.com/postcss/postcss, to selectively import functionality for use in CSS rather than the heavy-handed approach of bringing all of SASS, LESS, or Stylus. Leaving powerful expression to languages built for it seems natural -- especially when CSS and JS intermingle so heavily.
GUI toolkit and renderer development should begin with the humble admission that building a good GUI system is incredibly hard, on par with massive distributed systems and AI, and that the problem should be tackled with that level of difficulty as an expectation. A good analogy would be between a GUI toolkit and a mature 3d virtual world game engine. Those are probably roughly equivalent problem domains.
I'm just saying it's a problem domain that is consistently underestimated.
1) it's not surprising: a lot of very simple systems can compute. It seems to naturally happen as you add flexibility to your rules. See for instance Wang Tiles or some cellular automata.
2) it's not a good thing: it means that the behavior of CSS3 is undecidable in general, which makes it much harder to build tools that can meaningfully analyze it.