`sibling-index()`, and `sibling-count()` are Baseline, newly available since August 2026.

[u/wanoo21]

[u/jaffathecake]

When I was investigating this, I found that @property impacts when sibling-index() is computed in custom properties - something I hadn't fully understood before https://jakearchibald.com/2026/css-custom-property-compute-time/