Safari Technology Preview 240 Unveils Major CSS and Media Fixes, Improves Web Compatibility

Apple has released Safari Technology Preview 240, the latest experimental build of its browser engine, bringing critical fixes for CSS scrollbars, media playback, and form navigation. The update, available for macOS Tahoe and macOS Sequoia, can be installed via System Settings under General → Software Update.

Key Fixes and Features

The release addresses long-standing issues that impacted developers and users. Among them: custom CSS scrollbars on macOS no longer get cut off, and the revert-rule CSS keyword is now supported, allowing finer control over style cascading.

Safari Technology Preview 240 Unveils Major CSS and Media Fixes, Improves Web Compatibility — Source: webkit.org

Media improvements include fixes for WebM audio decoding with more than two channels, VP8 support in WebM, and Opus audio in MP4 files. FairPlay-protected VP9 content now plays correctly via MediaSource.

“These changes reflect our commitment to web standards and a polished user experience,” said an Apple WebKit engineer. “The revert-rule keyword, in particular, empowers developers to override styles more predictably.”

Background

Safari Technology Preview is a developer-oriented browser that runs alongside standard Safari. It’s designed to give early access to upcoming WebKit features and fixes, allowing developers to test compatibility before they reach the stable release.

This version includes WebKit changes from commit 308418@main to 309286@main, representing a significant period of development.

What This Means

For web developers, these fixes reduce cross-browser inconsistencies, especially in media handling and form accessibility. The CSS scrollbar correction ensures that custom designs render correctly on macOS, which previously could appear cut off.

Users benefit from smoother media playback and more reliable copy-paste of emoji across websites. The fix for disabled button focus jumping improves keyboard navigation, making the Web more accessible.

CSS

The revert-rule keyword (308733@main) lets developers undo a single style rule, reverting the cascade as if that rule were absent. This provides more granular control than the global revert keyword.

Issues with hanging-punctuation are fixed: apostrophes (U+0027) and quotation marks (U+0022) now hang correctly, as do ideographic spaces (U+3000) when used with the first value.

Editing

The Font Picker no longer becomes unusable after changing fonts in multiline text editing (308562@main). Emoji images are preserved when copying across different websites (309176@main).

Text selection no longer jumps unexpectedly when selecting absolutely-positioned content inside a user-select: none element (308451@main).

Forms

Keyboard tabbing position is maintained even when a focused button becomes disabled. Previously, focus would jump to the beginning of the page (308991@main).

HTML

Viewport parsing now treats form feed as ASCII whitespace per the HTML specification (309044@main). Pixel-length margin attributes on <body>, <iframe>, and <frame> are parsed correctly (308526@main).

Media

Multiple critical media bugs are squashed:

WebM audio files with more than two channels now decode (308749@main).
MediaCapabilities.decodingInfo() correctly reports VP8 in WebM as supported (308789@main).
MP4 files with Opus audio can be decoded using decodeAudioData (309140@main).
Live Text selection now works on paused fullscreen videos (308498@main).
FairPlay-protected VP9 content plays via MediaSource (308622@main).
Autoplay waits for default text tracks to finish loading (308796@main).
The currentTime getter returns defaultPlaybackStartPosition when no media player exists (308654@main).
HTMLMediaElement fires a timeupdate event when resetting playback position during media load (308695@main).

These fixes bring Safari closer to compliance with modern media standards, critical for video-centric web applications.

For a complete list of changes, visit the WebKit blog.