Inlining isn't necessarily a requirement for how it's used here. E.g. you could put something like:

  .container:nth-child(1) {--i: 1}
  .container:nth-child(2) {--i: 2}
  ...

In your CSS. Still not all that ideal given you need to ensure you have enough entries for all the entries you might have... but at least a more dynamic and self-contained option until the `sibling-index()` feature they mention roles out.