You know that thing where you have a grid of cards, and you want them to fade in one after another? That staggered cascade effect. Looks great. Should be simple. And yet every time I’ve built it, the implementation has made me feel like I’m doing something fundamentally stupid.

The current spec only counts all element siblings. But the CSSWG has documented a planned extension in issue #9572: an of <selector> argument, matching what :nth-child() already supports.

Something like sibling-index(of .active) would let you count only siblings matching a specific selector. An element that’s the eighth child overall but the third .active child would return 3. For dynamic UIs where you’re filtering or toggling visibility, that would keep the index sequential without requiring DOM manipulation.

There’s also been CSSWG discussion around children-count() and descendant-count() functions — the first would tell you how many children an element has (useful for parent-driven layouts), the second would count all descendants recursively. Both are still at the proposal stage, but they’d round out the tree-counting story: sibling-index() and sibling-count() give you the horizontal view (where am I among my peers?), while children-count() and descendant-count() would give you the vertical view (what’s below me?).

That feeling I mentioned at the top — writing ten :nth-child() rules for a staggered animation and wondering if you’re missing something obvious? You weren’t. The obvious thing just didn’t exist yet.