I'm well aware of details (I use it in GitHub READMEs frequently), but you still can't animate it like I described – without that, no designer is ever going to want you to actually use it. Builtin elements are worth fuck all if they don't meet your requirements!
You a moment ago:
> half the stuff things like React are used to do could be accomplished much better with a small amount of CSS
you now:
> yeah, you can't do it in pure CSS because you can't do most things in pure CSS
Well yeah, you can't do it in pure CSS because you can't do most things in pure CSS. The HTML element you're looking for is "details." (this seems to be one a lot of react people miss.)