CSS animations still lack a semantic way to sequence animations based on the beginning/end of some other animation, which SMIL offers. With SMIL you can say 'when this animation ID begins/ends only then trigger this other animation', including time offsets from that point.

Which is miles better than having to having to use calcs for CSS animation timing which requires a kludge of CSS variables/etc to keep track of when something begins/ends time-wise, if wanting to avoid requiring Javascript. And some years ago Firefox IIRC didn't even support time-based calcs.

When Chromium announced the intent to deprecate SMIL a decade back (before relenting) it was far too early to consider that given CSS at that time lacked much of what SMIL allowed for (including motion along a path and SVG attribute value animations, which saw CSS support later). It also set off a chain of articles and never-again updated notes warning about SMIL, which just added to confusion. I remember even an LLM mistakenly believing SMIL was still deprecated in Chromium.