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My understanding is, that in the end, it depends on if the reading agent supports your method or not. I've seen use of CSS, javascript, even embedding a GIF - but it didn't always work across reading environments.
Are you trying to create an effect as the result of some trigger, or begin immediately when rendered, etc.? What you want to happen may influence how/what you can use, and again in what readers it will actually work in.
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