What technology is used by sites like goanimate, xtranormal, and animoto? Especially the video compilation engine in the backend of animoto? Could HTML5 be used to do everything that's there?
Well, my copy of xtranormal (which is pretty old) has some Ogre3D related files in it, so I'm going to say its using either DirectX or OpenGL to render its graphics (Ogre apparently supports both), not flash. I also see a DirectX related DLL in there, so it sounds like, at least on Windows, xtranormal is using DirectX. On other platforms I'd assume OpenGL.
I took a look at goanimate, the progress bar I saw is the default Flex progress bar.
However, having a lot of experience with Flex and pure ActionScript projects, I would not recommend Flex for something like animations or games. Flex is more for business applications and RIAs.
I would recommend using Flash + ActionScript (3.0). I am sure that HTML5 can do most (if not all) that Flash can do, but it will be much harder (At least for now.)