Byte has an overview of how online video works, distinguishing between single-file, adaptive, and live streaming, as well as multicast and p2p. The article explains how the technologies differ on desktop and mobile devices.
Despite the hype over HTML5, it looks like Flash isn't going away any time soon. "For some Web producers, the technologies selected to deploy video is driven by religious zeal, which was evident when Apple launched the Flash-less iPad to the joy of plug-in haters everywhere. HTML5 went from obscurity to top of mind, and Flash was declared dead on dozens, if not hundreds, of websites.""
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