Improving HTML + CSS performance is absolutely one of the goals of Servo. Here's a video from pcwalton demonstrating reflow (HTML), repainting, and CSS animations on the GPU in Servo: https://air.mozilla.org/bay-area-rust-meetup-february-2016/#... (much of this code is being uplifted to Firefox as WebRender). It's true that Servo won't have any effect on Javascript performance, but WASM won't have any effect on HTML or CSS performance.