Layout is actually difficult and offers a great many interesting problems when you try to parallelize it or efficiently use GPU acceleration. There are a lot of constraints and text is difficult to size and render.
Layout is automated (and hopefully repeatable) typesetting.
The other thing Donald Knuth is famous for is getting so fed up with typesetting for his books, he created a document format to help him keep his sanity. And then went way way down that rabbit hole instead of finishing his book series. One might argue that's not sanity-preserving.
Agreed, layout is probably more difficult problem and definitely the area with more potential for improvements ("forced reflow" anyone?) whereas JS engines are already at the point of quickly diminishing returns.
An Intel GMA 950 can handle accelerating browsers. It's less about raw power and more about just being able to efficiently split up the workload to get the page rendered and visible in as little time as possible.
