See this demo comparing Chrome, Firefox and Servo WebRender: https://www.youtube.com/watch?v=u0hYIRQRiws Servo WebRender uses the GPU making it much faster than classic renderers (besides CPU parallelism), think of it like a Webbrowser using Video Gaming technology.
There's a talk about this at https://air.mozilla.org/bay-area-rust-meetup-february-2016/ (HN discussion: https://news.ycombinator.com/item?id=11175258)
browser.html is just the current "skin" for Servo.