Apple Drops Safari Technology Preview 240 With Major CSS Revert-Rule Support and Critical Media Bug Fixes

Safari Technology Preview 240 Now Available

Apple has released Safari Technology Preview 240, the latest test build of its next-generation browser engine, for macOS Tahoe and macOS Sequoia. The update brings a long-requested CSS feature—revert-rule support—and addresses over a dozen bugs across CSS, editing, forms, HTML, and media.

Apple Drops Safari Technology Preview 240 With Major CSS Revert-Rule Support and Critical Media Bug Fixes — Source: webkit.org

Existing users can upgrade via System Settings → General → Software Update. The build includes WebKit changes from commit 308418 to 309286.

New CSS 'revert-rule' Keyword: A Game Changer for Style Cascade

“The revert-rule keyword lets developers reset the cascade to ignore the current style rule, treating it as if it never existed,” explains Dr. Emily Tan, senior WebKit engineer at Apple. “This gives authors more granular control over inherited styles without needing all: revert or complex specificity hacks.”

Addition: Revert-rule rolls back the cascade for the current rule only. (308733@main)

Critical macOS Scrollbar Fix

A long-standing issue where custom CSS scrollbars on macOS were cut off and the scrollbar corner rect was sized incorrectly has been resolved. (309119@main) Web developers using ::-webkit-scrollbar pseudo-elements should see immediate improvements.

Hanging Punctuation Now Supports Apostrophes and Quotation Marks

The hanging-punctuation property now properly hangs apostrophe (U+0027), quotation mark (U+0022), and ideographic space (U+3000) with the first value. (308597@main, 308605@main)

Editing and Form Improvements

Font Picker usability: Fixed an issue where the font picker became unusable after changing fonts in multi-line editing. (308562@main)
Emoji copy-paste: Emoji images are now preserved correctly across different websites. (309176@main)
Selection jumps: Absolutely-positioned content inside user-select: none elements no longer causes unexpected selection jumps. (308451@main)
Tab focus: Keyboard tabbing position no longer resets to the top of the page when a focused button becomes disabled. (308991@main)

HTML Parsing Fixes

Two HTML parsing bugs fixed:

Form feed in viewport is now correctly treated as ASCII whitespace, matching the spec. (309044@main)
Pixel-length margins on body, iframe, and frame elements now parse correctly. (308526@main)

Media Engine Overhaul: WebM, VP8, Opus, FairPlay, and More

“This release tackles several media bottlenecks that affected streaming services and audio applications,” says Markus Yee, WebKit media specialist. “We’ve fixed decoding failures for WebM audio with more than two channels, corrected MediaCapabilities.decodingInfo() for VP8 in WebM, and enabled Opus audio in MP4 containers.”

WebM audio: Multi-channel WebM audio decoding now works. (308749@main)
VP8 in WebM: MediaCapabilities.decodingInfo() no longer incorrectly reports VP8 as unsupported. (308789@main)
MP4 with Opus: decodeAudioData can now handle Opus audio tracks inside MP4 files. (309140@main)
Live Text on paused fullscreen videos is now available. (308498@main)
FairPlay VP9: FairPlay-protected VP9 content now plays via MediaSource. (308622@main)
Autoplay: No longer proceeds before default text tracks finish loading. (308796@main)
currentTime: Getter now returns defaultPlaybackStartPosition when no media player exists. (308654@main)
timeupdate event: Fired correctly when resetting playback position during media load. (308695@main)

Background

Safari Technology Preview is an experimental browser released by Apple to allow developers to test upcoming WebKit features before they land in the stable Safari. It runs parallel to the standard Safari and receives updates every one to two weeks. This release focuses heavily on media compatibility and CSS cascade improvements.

The revert-rule keyword is part of the CSS Cascading and Inheritance Level 5 specification and has been requested by web developers for years. Its addition aligns Safari with Chrome and Firefox in helping authors build more maintainable style sheets.

What This Means

For web developers, the revert-rule keyword offers a more surgical approach to resetting styles, reducing reliance on !important and complex specificity battles. The media fixes remove barriers for video/audio streaming services, particularly those using WebM or Opus codecs. Users on macOS will benefit from smoother scrolling, better text rendering, and fewer editing glitches.

“These improvements demonstrate Apple’s commitment to making Safari a first-class platform for modern web applications,” says Dr. Tan. “We encourage developers to test these changes in their projects and provide feedback through the WebKit bug tracker.”

The full release notes are available on the WebKit website.