They're different standards with some overlapping application. Video and Rich UI are where html5 and flash seem to overlap the most to me. Video is the most controversial point due to many reason and it's the main point of this thread. html5 will bring improvements to whole sites, not just selected flash elements on a web page. It has offline support, geolocation support, etc.... and it can also be programmed to support GPU acceleration (MS makes a big deal of this on their upcoming IE9).