Berry de Vos has created a demo of how animated backgrounds (built with canvas) would work. The animated background consists of scrolling bars which slowly change angle and colour over time.
Berry has these hints on using a large canvas:
When I created the first setup for this demo I started by setting the height and width of the Canvas to the height and width of the page. This soon turned out to be quite heavy on the CPU usage at higher resolutions. So I made the Canvas a static size and let the browser resize the area to 100%. This works and performs way better.
Works best in Chrome.