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Two new CSS functions. sibling-index() gives an element’s position among its siblings, starting at 1, and sibling-count() gives how many there are. Both return plain numbers you can use in calc().

li { animation-delay: calc(sibling-index() * 80ms); } That works in every current run. A staggered animation on a list that does not change is the one thing you can do with this today.

The trouble starts when the index is read somewhere other than a plain declaration. Safari 26.6 passes 3 of the suite’s 22 tests whole: it fails the index inside a gradient, inside a container query, and inside a shadow tree.