The img element must not be used as a layout tool.
If an image is decorative but isn't especially page-specific — for example an image that forms part of a site-wide design scheme — the image should be specified in the site's CSS, not in the markup of the document.
Afair, something like that is missing in the specification of the canvas element:
The canvas element provides scripts with a resolution-dependent bitmap canvas, which can be used for rendering graphs, game graphics, or other visual images on the fly.
One could interpret that "visual images" means decorative images, too. So one could argue that using canvas for a background animation (like sparkling stars on a blog about frogs; so this animation is not "content") would be allowed.
But it isn't, is it? canvas should be used for content images only, right?