A surprising amount of stuff that you might think it's done with just dumb bitmap rendering surfaces, is these days actually painted on graphics primatives with it all composited together by your GPU.

It likely isn't using much more power than if running at entirely idle, often enough less than the extra a CPU might use doing the same job at a lower frame rate, displaying and updating your current desktop.

Why does it worry you? GPUs are designed to push pixels around. This a perfect use of them.

Most browsers are GPU accelerated.