If you can use flash movie as background with CSS? Let say I have a CSS based website but I want to use a flash background that does some simple animation.
you could use SWFObject's 100% width and height Flash embed method with the "wmode" parameter set to "transparent" and then absolutely position a DIV on top of the Flash, although if the content is larger than one screen, you won't be able to scroll (I think, you'll have to test to be sure).
That's what Amazon.com is doing right now (sort of) for its Christmas promotion. Animated snow in a Flash movie with menu elements dropped on top using z-leveling. They only are doing the background of their menu, not the entire page.
It can be done, but from a usability standpoint I would avoid it. I find those things distracting and annoying, not to mention how it increases load times for the page which is terrible for users on slower connections.