I like the writeup and drawings, but I have to mention that this is a bit too specific to browsers as they work right now, specifically Chrome. Having to pay attention to the paint/compositor distinction and layer count is something that newer renderers like WebRender make obsolete. You don't have to worry about layer count causing needless overdraw if you have a unified system so that occluded content is culled, for example.

WebRender is already turned on by default for many users in Firefox Nightly…