I addressed your first paragraph in another comment, but for the second one, compositing is currently on GPUs in most browsers. Layers are painted on the CPU an

I addressed your first paragraph in another comment, but for the second one, compositing is currently on GPUs in most browsers. Layers are painted on the CPU and uploaded as textures to the GPU, which are then composited together. Manipulation of a few properties can be done cheaply in the compositor (modifying the layer transform or opacity for example). And that is assuming that you perfectly layerized, which is a heuristic process. You can find plenty of web dev articles about how to fix your CSS so that it layerizes propertly or how to avoid animations and transitions that can't be accelerated in the compositor. Another savings (memory, time, unsure about energy) comes from not having to upload giant pixel buffers to the GPU.