It sounds like a weird joke doesn't it?
Anyways, ass.js seems to be a relatively mature project that does what it says on the tin, but I can't say I know how hard it would be to integrate into the existing code.
This feature would be nice because as far as I can tell there is no (existing!) way to go from a ASS subtitle file to WebVTT that does more than just the plain text. ASS is super common in anime because it can do nice things like move text across the screen, blur/cover parts of the screen (to cover text and put translated text on top of it), and while this is all in theory accomplishable with WebVTT, nobody has implemented a converter.