I would strongly recommend a black background to go with the blackboard theme of the videos. (Alternatively, switch to whiteboard visuals). It would blend so nicely. Also, the blackboard animation should be in a smooth vector format instead of blurry compressed, bandwidth-intensive videos. Now for words and symbols, I recommend text. You can easily time-release it with JavaScript. For diagrams, maybe something like Graffiti Markup Language. http://www.graffitimarkuplanguage.com/about/