I'll admit I'm certainly not an expert on browser rendering pipelines, but that's not the impression the article gave. Rather, it suggested that current browser engines may, in fact, be more efficient in terms of the absolute number of computations done.

When discussing existing browsers:

> But often the things on these layers didn’t change from frame to frame. For example, think of a traditional animation. The background doesn’t change, even if the characters in the foreground do. It’s a lot more efficient to keep that background layer around and just reuse it.

> So that’s what browsers did. They retained the layers. Then the browser could just repaint layers that had changed.

Then, later in the article, when discussing WebRender:

> What if we removed this boundary between painting and compositing and just went back to painting every pixel on every frame?

(Emphasis mine.)

So is WebRender less efficient in terms of power usage? Or is there some other factor that offsets the cost of this extra work?