More cores =/= better performance, it’s also about how well they can scale on their own and when they’re workig with other cores.
You’ve changed your argument slightly. You suggested splitting CSS over multiple cores (again, as-per your example) which is extremely difficult. What you’re saying now incorporates what I was trying to say earlier with putting CSS on one core, JS on another, etc.
Don’t forget that not everyone has CPUs with many cores. And also, only AMD’s Opterons based on “Bulldozer” (and its derivatives) have 16 cores on a chip. Hyper-Threading does not count as extra cores, only extra hardware threads which, again, may or may not be a good thing.