Just recently I had to code a little but complex animation and I ended up using the web animations API. It's nice, it was a lot less code than the css counterpart.

But, if I* have to write this same animation in two years and @property is widely available, I'll reach up for that first.

*To be honest, it will be 100% an LLM that writes it for me.

Which are CSS animations underneath

Yes, but you define them with Javascript rather than in CSS which is a key difference since the syntax is more intuitive for some (myself included)

So since the original comment was questioning the necessity of animations in CSS, and JS is essentially using CSS animations for many things, I'd say the answer is yes. It's also a great way for UI designers to use subtle movement to hint at actions/status/next steps/etc without having to execute any JavaScript. When you've got a purely visual task, I'd say involving a general purpose programming tool is the unnecessary part— like using sed or the like vs opening up a repl in Python or whatever.

No - CSS and JavaScript are two ways of defining the same thing, an animation. Safari runs its accelerated animations with CoreAnimation - is CoreAnimation a CSS animation?