Where are the HTML5 web adverts? They don't exsist because:
1)you have to code things 4 times (once for all the browsers + fallback)
2) There are no artist friendly tools (google's flash translator only works for chrome, see above)
3) Everything looks different in each browser
4) JS animation is Sllllloooooowww
5) Animation primitives are missing, those that exsist are from the 1960s
6) there are no mature standard/implementations (in widescale use)
with flash's workflow you:
1) create a flash file with the mature tools, test, release
2) create an animated gif as fallback
yes its buggy, and the video is slower than decent native support, but it works the same across all browsers (90%)
On a 2010 macbook that runs at 15fps, which is a great step forward in speed for moving 8 sprites. Have you actually tried to animate anything other than a solid body, in just code?
http://www.idleworm.com/how/anm/02w/walk1.shtml is a walk cycle, and as you can see they are animating the control points, so the body smoothly deforms. As far as I'm aware you can control individual control points of an SVG with javascript. Even if you could without a decent interface you couldn't do more than move a ball about.
HTML/JS is shit at animation.
Flash is rubbish, and can be much more efficient, however HTML/JS is 10 times worse.
Also optical flow really doesn't handle occlusions very well, so if one thing obscures another you'll get lots of artifacts.