I am too young to have lived through that era, but it seems to me like tweening hints were all integrated in the editing UI? That is of course much more useful. But the scope of my post was not at all to do something better than Flash, or to integrate a full editor. It's just an explanation for one way to achieve it, leveraging what SMIL already offers.
Yeah, they were in the editor and buggy as hell and you ran an increasing risk of crashing the tool the more of them you used. Also IIRC, they were indexed by letter of the alphabet and everything became impossible > 26.
