It seems like a valid point that many of the details of HTML and CSS, while you can use all kinds of fancy techniques to make them a little faster, impose limits on performance.
It's true that the DOM is a bottleneck on many apps (I don't think anybody is disputing that), but "a little faster" undersells what Servo is trying to achieve: order-of-magnitude speedups on GPU-parallelizable tasks, and at least factor-of-two speedups on CPU-parallelizable tasks, all while providing stronger guarantees WRT user security than any contemporary browser.