What’s !important #16: sibling-index() Animations, Use Cases for the infinity Keyword, Container Stuck Queries, and More

On August 18 2026 Firefox 154 ships the long‑awaited sibling-index() function, a Baseline feature that lets developers target elements based on their position relative to a sibling for animation timelines. Temani Afif demonstrated the technique with slick demos, proving the API can replace JavaScript‑heavy sequencing in many UI patterns. At the same time, Chris Coyier highlighted container‑scroll‑state queries that expose whether an element is “stuck” inside a scroll container—a capability still missing from Safari and Firefox, underscoring a split in container‑query adoption across browsers.

The roundup also revisits niche CSS tools that are gaining traction. Adam Argyle’s deep dive into the calc() infinity keyword shows practical uses such as turning superellipse corners into perfect squares or notches, expanding the design vocabulary without extra assets. Lea Verou’s call for a global lazy‑loading default, referencing the defunct @tabatkins.com CAS proposal, adds pressure on standards bodies to codify image‑ and video‑loading behavior. Meanwhile, Johannes Bechberger’s three SVG‑based hand‑drawn text animations illustrate how emerging CSS‑animation primitives, like sibling-index(), can simplify traditionally complex SVG work.

These developments signal a maturing CSS ecosystem where browsers compete to be the first to ship declarative layout and animation tools, while the community pushes for broader defaults that improve performance. The biggest risk is fragmentation: container‑stuck queries and sibling-index() are useful now only in Firefox, leaving developers to write fallbacks for Chrome, Safari, and Edge. Watch the upcoming Firefox 155 release notes for any cross‑browser alignment, and monitor the W3C CSS Working Group’s progress on standardizing lazy‑load defaults and container‑scroll‑state syntax, which could force other vendors to catch up before the Chrome roadmap reaches version 500.