Context2D is slow and lacks a lot of features present in flash. It's not even really close. If you're willing to put in the (large amount of) legwork, WebGL so

Context2D is slow and lacks a lot of features present in flash. It's not even really close. If you're willing to put in the (large amount of) legwork, WebGL solves this, but it works on far fewer machines (even assuming up to date browsers) than flash -- well, if you discount mobile at least, not that WebGL on mobile works very well across the board. Flash also was able to give you much better guarantees about things looking identical across browsers. This takes a lot more work when doing game UIs in HTML/CSS, to the point where my recommendation would be not to use HTML/CSS for games where this matters. HTML5 audio element is very unreliable (it plays when it feels like) and limited. WebAudio theoretically should be better, but in practice you get pops and skips fairly frequently (depends on browser, etc). There are other things too, but it's been a while since I did any work in flash/actionscript. Those are the ones that immediately come to mind.