Well... failure

I spent three and a half hours trying to figure out what's going on and why. I simply do not understand it. WEBM support is far more limited when trying to chain it, what I don't understand is why. It most likely is due to its "flow" being controlled using sound channels instead of the usual Displayable timer and some form of desynchronisation takes place.
===>>
I might be worth looking into writing our own UDD class somehow. I did manage to improve the attack, it actually doesn't look bad at all but it's nowhere near what it should be like (I've tried about everything I could think off and plainly not every frame of the movie is being played, even if only one movie is played by the class, that sh1t is particularly weird... there may be a simpler solution than trying to rewrite Movie class but I don't know what it is (yet)).
Edit (at almost 4 AM):
Nope... there is no "writing" our own UDD, syncing is too heavily integrated with sound channels

There may be some form of workaround but I doubt it.
Our best chance for chaining WEBM is our ATL Attack class and custom scenarios for each animation. WEBM also would have to be in infinite loop for this one. It's far from perfect solution and it may be time consuming to write but there is no other option for chaining them in perfect sync

Or, if it looks ok, multi attack class may still offer something on case per case basis...