The amount of work required to do so is epic, and considering the rate of Flash's evolution it's like getting on a treadmill whose speed is controlled by Adobe.
Funny metaphor, and very accurate. However, HTML5 is like that same treadmill, only that people still have to build the treadmill itself. It's epic work times two.
HTML5 is mostly based on existing de facto standards. If you stick to those, you're good with HTML5.
That standard is for Flash as it existed several years ago AFAIK. It's not useful for creating a viable competitor to modern Flash except maybe as a jumping-off point to the actual hard work.