Apple has released Safari Technology Preview 254, available for macOS Golden Gate and macOS Tahoe. The preview build incorporates WebKit changes spanning revisions 321068@main to 322634@main, bringing new CSS layout functions, WebCodecs memory optimizations, and stability fixes across accessibility, graphics, and JavaScript standards.
Dynamic Intrinsic Sizing in CSS
Safari Technology Preview 254 adds support for the CSS calc-size() function and the interpolate-size property. The calc-size() function allows developers to perform mathematical calculations directly on intrinsic sizing keywords such as auto, fit-content, and max-content. Combined with interpolate-size, web applications can now animate transitions to and from intrinsic layout dimensions that previously could not be smoothly interpolated.
The release also introduces size, min-size, and max-size shorthand properties, alongside border-*-radius side shorthands like border-top-radius to adjust adjacent box corner radii simultaneously. Additional styling updates include the symbols() function for anonymous counter styles and CSS linked parameters, enabling values to be passed into SVG graphics using link-parameters and param().
WebCodecs and Media Pipeline Enhancements
For web media applications, WebCodecs constructors—including EncodedAudioChunk, EncodedVideoChunk, AudioData, VideoFrame, and ImageDecoder—now accept a transfer option. This feature allows ownership of underlying ArrayBuffer binary data to be transferred directly between contexts instead of copied, avoiding unnecessary memory duplication during real-time media processing.
In addition, VideoEncoder configurations now support an hevc option to produce HEVC video streams in Annex B format. WebKit has also updated its software VP9 decoding to correct chroma channel mapping in 8-bit 4:2:2 content and prevent silent frame drops when handling 4:4:4 formats.
Layout Engine and JavaScript Fixes
This preview release resolves multiple long-standing issues in CSS Grid and text layout. Grid track sizing logic was updated to calculate multi-track spans accurately without triggering quadratic relayout overhead, while line-clamp rendering was revised to align with updated specifications for block ellipsis placement and line counting in nested blocks.
JavaScript runtime updates address cross-realm object handling, ensuring that promises, regular expressions, and ArrayBuffer instances maintain their native constructors when executing across iframes. Internationalization formatting fixes resolve output errors in Intl.DurationFormat and Intl.Segmenter, and VoiceOver accessibility improvements resolve issue tracking for custom controls styled with pointer-events rules.
AI-generated editorial illustration; not a documentary photograph.
Source: Release Notes for Safari Technology Preview 254
SIGNAL TITANS
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