As it seems, running animations in WebGL should have the best performance.
I saw that the webgl export is still in beta, when is webgl expected to be fully working.
And I noticed that when creating a WebGL document I can't use bones, but when I convert an existing document (copy paste layers) with bones, they nicely convert to separate keyframes.
Why can't I use bones in a webgl document, and convert them on export ? Why am I restricted as I choose my export format.
I agree that it would be nice if you could use all of the features and then only lose the ones that can't work when you export. If you're just doing animation you could work that way yourself, in that you could create the complete animation with all the tools you get in an ActionScript 3.0 FLA, and when you're ready you would use the Convert to Other Document Types option in the Commands menu. That would then turn off anything that doesn't work, and you could go around adjusting just what you need to do to make it work for WebGL.
I would take the term "Preview" more optimistically. It's not saying that it's beta, but it is suggesting that the features will continue to improve.
Depending on the type of animation you're doing, WebGL may not be the best option. If there are a lot of unique symbols you could fill up GPU memory to the point that it then plays very slowly, or not at all. If you used the AS3 type for as long as you can, you could then try both HTML5 Canvas and WebGL, and decide quite late on which works best.
Also, the AS3 approach would happen to give you a perfect version of the animations that could be used in mobile apps or by desktop browser users.