Safari Technology Preview 251 adds corner-shape, and the first way to feature-detect an at-rule

Safari Technology Preview 251 came out on August 26 for macOS Golden Gate and macOS Tahoe, covering WebKit changes between 317935@main and 319386@main. It is a large release for CSS, with twelve new features, and the headline item is not the one that will get the screenshots.

The quiet one is at-rule detection

WebKit added support for @supports at-rule(...), which tests whether a CSS at-rule is supported. Feature detection has covered properties and values for a decade, but never at-rules. That gap is the reason @container, @scope and @layer have had no honest progressive-enhancement story: you could test a property inside a rule and infer from that, or you could sniff a side effect, but you could not simply ask whether the browser knows the rule.

This matters more than it sounds. A stylesheet that can ask gets to ship a real fallback branch instead of a guess, and the guess is where the bugs have been living.

What else arrived in CSS

The corner-shape property is the one that will get attention, because it turns squircles and notched corners into two lines of CSS instead of an SVG mask. Alongside it: object-view-box for framing replaced elements, white-space-trim inside the white-space shorthand, external and data: URL filter references on HTML elements, and HighlightRegistry.highlightsFromPoint().

Three additions belong together. ident() composes a custom property name, inherit() reads an inherited value, and random() now works inside custom properties, with property-scoped and property-index-scoped caching keywords that decide when the value re-rolls. A new random-item() function picks from a list. Between them, a chunk of what used to require a small script becomes declarative.

@container queries now take a comma-separated list of conditions, which removes the duplicated block that every container-query codebase currently carries.

JavaScript: iterator helpers, round two

The iterator work continues with Iterator.prototype.includes(), Iterator.prototype.join(), chunking through Iterator.prototype.chunks() and Iterator.prototype.windows(), and joint iteration through Iterator.zip() and Iterator.zipKeyed(). These are the methods that let you stop materializing an array in the middle of a pipeline.

Also new: deferred module evaluation with import defer, importing text modules with an import attribute of type: text, WebAssembly Memory64, the traceStack option on WebAssembly.Exception, the RegExp Buffer Boundaries proposal, and era and monthCode fields in Intl and Temporal calendars. The resolved-issues list is unusually heavy on regular expressions, with fixes for lookbehind, negative lookahead, sticky patterns wrapped in .*, and anchoring inside a group that can match empty.

What this is not

Safari Technology Preview is a preview build. Checked on August 27, 2026, none of these features are available in shipping Safari, and a feature landing here carries no Baseline status and no release date. Treat the list as a view of what WebKit is working on, not as something you can put behind a support query in production this quarter.