Safari TP 251 gives CSS a way to ask whether an at-rule exists

The useful part of `@supports at-rule()` is where it sits in the dependency chain. According to the WebKit release notes it tests whether a CSS at-rule is supported, landing at changeset 318482@main ^[3], which is exactly the check you want before committing a component's layout to `@container`. But the test is a feature like any other, so it only answers in an engine that already parses it. The notes say WebKit has it; they say nothing about anyone else. Until that changes, the branch that actually runs in the field is the fallback, and the detection branch is the one you cannot verify from a stylesheet.

The comma-separated condition list in `@container` ^[7] is duller and far more likely to touch code this quarter. A design system that queries containers accumulates near-duplicate declaration blocks, one per condition, because the query head could hold only one. Collapsing those is a maintenance change, not a new capability, and it is the sort of change that quietly reduces the number of places a spacing token has to be edited.

The same build also corrects two things inside `@container`: style queries were evaluating root-relative lengths incorrectly ^[17], and `::part()` in a container query had the wrong originating element, which is now the element itself ^[18]. Both are the kind of defect that leaves a component looking right in isolation and wrong once it is dropped into a page. They also say something about maturity: the syntax is expanding while the evaluation is still being fixed.

Three of the twelve new CSS features concern `random()` ^[16]: the `random-item()` function ^[6], `random()` inside custom properties ^[8], and two caching keywords, `property-scoped` and `property-index-scoped` ^[5]. The keywords are the load-bearing part. Their names describe the granularity at which a generated value is held rather than regenerated, and without a stated scope a random value in a stylesheet is not something you can screenshot-test.

For most of the rest, the notes give a name, a changeset and a Radar ID, and stop there: `object-view-box` ^[4], `corner-shape` ^[11], `ident()` ^[9], `inherit()` ^[10], `white-space-trim` in the `white-space` shorthand ^[12], external and `data:` URL `filter` references on HTML elements ^[13], and `HighlightRegistry.highlightsFromPoint()` ^[14]. If you want semantics rather than surface, the WebKit range 317935@main to 319386@main ^[2] is the only documentation on offer, alongside a CSS fix list of at least twenty-five entries covering nested `@scope`, flex baselines, scroll snap ordering and `var()` substitution ^[20][21].

One non-CSS item is worth pulling out. SVG anchors without an `href` were being exposed as links, and are not any more ^[19]. That bug never appeared in a layout diff. It appeared in what a screen reader read out.