The paint phase of rendering can easily happen on the GPU, and more and more stuff will move to the GPU as time goes by. The style and layout phases… unlikely.
Think of a GPU this way: If you could reasonably launch a thousand threads on the CPU and get a benefit from it—i.e., essentially, you have tens of thousands of largely independent work chunks (or more)—your problem might be well-suited to a GPU. A large web page contains a couple thousand DOM elements, and many of them depend on each other wrt. style, so it's not a good fit. (Stylo is parallelized, so it is capable of using a multicore CPU, but there's no way it can get a reasonable gain from 1000 cores.) Layout is also nontrivial to parallelize.
However, paint is a different beast. In many cases, you can do per-pixel parallel work. So various forms of font work, line/shape rendering, compositing/blending, scrolling, video decoding… that's where the GPU is useful.
Think of a GPU this way: If you could reasonably launch a thousand threads on the CPU and get a benefit from it—i.e., essentially, you have tens of thousands of largely independent work chunks (or more)—your problem might be well-suited to a GPU. A large web page contains a couple thousand DOM elements, and many of them depend on each other wrt. style, so it's not a good fit. (Stylo is parallelized, so it is capable of using a multicore CPU, but there's no way it can get a reasonable gain from 1000 cores.) Layout is also nontrivial to parallelize.
However, paint is a different beast. In many cases, you can do per-pixel parallel work. So various forms of font work, line/shape rendering, compositing/blending, scrolling, video decoding… that's where the GPU is useful.