I think of canvas widgets as being sort of like <canvas> elements on web pages: a drawing surface that contains one image at any given time and which can be manipulated with scripts.
Building on what Ahm said, it's certainly possible to create contraptions that bundle together a canvas and storage for a sequence of animation frames, with logic for playing those sequences back. The WigglyKit deck comes with a contraption along these lines called wigglyPlayer.
Here's a little demo video of using a WigglyPlayer to create looping animated "sprites" on top of a drawing, with no custom scripting required:
