I was interested in doing short animations of my pictures, and occasionally I get asked to make ecards. With Flash not working on the iPhone / iPad, and mp4 not working in Internet Explorer and Firefox, there seems to be no good solution (I still use Gifs for quick ecards). There is a great article about online video here that covers pretty much everything:
From that page, the best solution is:
Wrap the ‹object› tag for Flash video inside the ‹video› tag, and make both an .mp4 and an .ogv file available on your server. This will work with damn near everything, since the browser in question will use whatever it can support.
That means you need to have your animation in three different formats….