macOS 27.3 Beta 1 is Out--Build 26C5035r Puts Vertical Tabs and Adaptive Dark Mode in Safari, Renders Markdown in Quick Look, Opens Siri AI to Any Developer's App, and Warns When a Printer Driver or Audio Plug-In Still Needs Rosetta

2026November 4, 2026Macs Only!Mac OS
A MacBook Pro screen showing Safari in macOS 27 on a baking site's Basque cheesecake recipe, with a Describe an Extension dialog in front listing Boost Productivity, Improve Focus, Get Creative and Design & Develop above a text field that reads "Save and track cooking recipes from around the web" and Cancel and Next buttons; the menu bar clock reads Wed Apr 1 9:41 AM and the Dock runs along the bottom.
Photo: Apple · A MacBook Pro screen showing Safari in macOS 27 on a baking site's Basque cheesecake recipe, with a Describe an Extension dialog in front listing Boost Productivity, Improve Focus, Get Creative and Design & Develop above a text field that reads "Save and track cooking recipes from around the web" and Cancel and Next buttons; the menu bar clock reads Wed Apr 1 9:41 AM and the Dock runs along the bottom.

macOS 27.3 beta 1 is out for developers. Apple's developer releases page lists macOS 27.3 beta as build 26C5035r, dated November 3, one week after macOS 27.2 shipped on October 26, as MacRumors' report on the beta notes. Where the first 27.2 beta came with notes that listed no features, this one declares its payload on day one: vertical tabs and a dark mode for bright web pages in Safari, Markdown rendered in Quick Look, Siri AI opened to any developer's app, and a Rosetta notice for printer drivers and audio plug-ins.

Safari gets vertical tabs and a dark mode for bright sites

Apple's Safari 27.3 beta release notes, dated November 3 and numbered 20626.1.12, carry two features a user will see among the engine changes: Vertical Tabs, "a new vertical layout for navigating and managing tabs with a space-efficient design that maximizes webpage content," and Adaptive Dark Mode, "which automatically adapts bright web pages that don't natively support dark mode." MacRumors' report on the Safari changes has the particulars: the tabs sit at the left edge as icons, the bar can be widened to show titles, and on the Mac the switch is in Safari's View menu. When you use Dark Mode and reach a bright page, Safari offers to "Try" the dark mode; it then "makes the background darker, changes text contrast, inverts logos and graphics, recolors vector artwork, lifts raised surfaces, and rebalances layered shadows," site by site or everywhere, and a site can add a tag to opt out. MacRumors says the release comes in December after several rounds of betas, and the notes list the same Safari beta as available for macOS 26 and macOS Sequoia, the systems the Intel Macs are on.

Siri AI opens to every app, and its skills are Markdown files

Apple's developer news post of November 3 says that with the 27.3 betas "a new Developer Preview enables any developer to integrate their app with Siri AI," through App Intents, with "skills" for requests that chain several actions, and a Siri that "can now surface any entity in the Spotlight semantic index." The preview reaches customers "in a few months," App Store submissions open "in the spring," and Siri AI "will soon expand to support Traditional Chinese, Norwegian, Turkish, and Vietnamese," the four languages our October 27 report on 27.2 had Apple promising by the end of the year. Apple's documentation for the preview adds that no new entitlement is needed and that the integration does not reach external TestFlight testers or App Store builds while the preview lasts. 9to5Mac's report notes that a skill is a SKILL.md file, and that 27.3 renders .md files as formatted text in Quick Look rather than the raw syntax.

Rosetta gets louder about plug-ins and printer drivers

Apple's macOS 27.3 beta release notes are a developer document, but two entries under Rosetta reach every user. The system "now notifies users when an audio plug-in or print dialog extension that requires Rosetta is used, and lists these plug-ins in the Intel-based apps list in General settings," and it "now notifies you when you print using a printer driver that requires Rosetta but is not yet updated for Apple silicon," and recommends AirPrint instead. That is the countdown our September 15 report on Golden Gate described, Rosetta's last full release, made visible one plug-in at a time. Also fixed: Macs with M5 Pro, M5 Max and M5 Ultra chips booted Linux "from a custom raw boot object" with a single core.

What to do now

Registered developers get the build from System Settings, General, Software Update, by opening the Beta Updates menu and choosing the macOS Developer Golden Gate Beta, per 9to5Mac's steps, after a full backup; our standing advice is a spare Mac. For a Mac that stays behind, 9to5Mac's report on the release candidates has macOS Sequoia 15.8.3 as build 24H314 and macOS Tahoe 26.7.3 as 25G416, with notes that say only "security fixes for your Mac," a week after 15.8.2 and 26.7.2. We'll post a hands-on report after we install it.

Then and now

On November 4, 1997, we asked one question in our note on Mac OS 8.1: QuickTime 3.0 had slipped to January, so had the point release that was to carry it slipped too? Our November 27 item relayed NEWS.COM's answer, early January and free to Mac OS 8 owners, and our hands-on report of February 1, 1998 found HFS+ and Open Transport 1.3 in the January 19 download "but not QuickTime 3.0." Twenty-nine years on, the point release's features are in Apple's notes the day the beta appears. Vertical tabs took Safari long enough; we'll take them.

Sources

  1. Apple Developer News, “Releases - Apple Developer” accessed Nov 4, 2026The releases list for November 3, 2026: macOS 27.3 beta (26C5035r), Xcode 27.3 beta (27C5020n), iOS 27.3 beta (24C5037i), iPadOS 27.3 beta, tvOS 27.3 beta, visionOS 27.3 beta and watchOS 27.3 beta, all dated November 3, 2026; macOS 27.2 (26B123) dated October 26, 2026; accessed November 4, 2026.
  2. Apple Developer Documentation, “macOS 27.3 Golden Gate Beta Release Notes | Apple Developer Documentation” accessed Nov 4, 2026The two Rosetta entries: "macOS now notifies users when an audio plug-in or print dialog extension that requires Rosetta is used, and lists these plug-ins in the Intel-based apps list in General settings" and "macOS now notifies you when you print using a printer driver that requires Rosetta but is not yet updated for Apple silicon. macOS also prompts you to install Rosetta if it isn't already installed. To avoid notifications, use AirPrint"; the mBoot fix: "Only a single core is available when you boot Linux or other third-party operating systems from a custom raw boot object on Mac computers with M5 Pro, M5 Max, and M5 Ultra chips"; accessed November 4, 2026.
  3. Apple Developer Documentation, “Safari 27.3 Beta Release Notes | Apple Developer Documentation” accessed Nov 4, 2026"Released November 3, 2026 — 27.3 beta (20626.1.12)"; "Safari 27.3 beta is available for iOS 27.3 beta, iPadOS 27.3 beta, visionOS 27.3 beta, macOS 27.3 beta, macOS 26, and macOS Sequoia"; under Browser, "Vertical Tabs introduces a new vertical layout for navigating and managing tabs with a space-efficient design that maximizes webpage content"; under Rendering, "Added Adaptive Dark Mode, which automatically adapts bright web pages that don't natively support dark mode, giving users an optimized viewing experience when browsing at night or in dark environments"; accessed November 4, 2026.
  4. Apple Developer News, “Siri AI is now open to all developers - Latest News - Apple Developer” accessed Nov 4, 2026November 3, 2026: "Starting today with the iOS 27.3, iPadOS 27.3, macOS 27.3, and visionOS 27.3 betas, a new Developer Preview enables any developer to integrate their app with Siri AI"; integration "by using App Intents"; "we're also introducing skills to let you teach Siri AI how to combine several actions into a single experience"; "Siri AI can now surface any entity in the Spotlight semantic index"; "early access before first shipping to customers in a few months; and with submission to the App Store opening up in the spring"; Siri AI "will soon expand to support Traditional Chinese, Norwegian, Turkish, and Vietnamese"; accessed November 4, 2026.
  5. Apple Developer Documentation, “Siri AI Developer Preview | Apple Developer Documentation” accessed Nov 4, 2026"You don't need to add a new entitlement or join a separate developer program"; "The Siri AI Developer Preview isn't available to external testers in TestFlight or to versions published on the App Store"; accessed November 4, 2026.
  6. MacRumors, “Apple Releases First macOS Golden Gate 27.3 Beta - MacRumors” accessed Nov 4, 2026Juli Clover, November 3, 2026, 10:10 a.m. Pacific: the first macOS Golden Gate 27.3 beta, build 26C5035r, one week after macOS Golden Gate 27.2; accessed November 4, 2026.
  7. MacRumors, “iOS 27.3 Adds Vertical Tabs and Adaptive Dark Mode to Safari - MacRumors” accessed Nov 4, 2026Juli Clover, November 3, 2026, 10:51 a.m. Pacific: Vertical Tabs on the left side, icons by default with an option to widen the bar to show website titles, with "the option to turn on Vertical Tabs" in Safari's settings "or in the View menu on Mac and iPad"; "If you use Dark Mode and then browse to a website with a bright background color, Safari will offer an option to 'Try' Adaptive Dark Mode"; it "makes the background darker, changes text contrast, inverts logos and graphics, recolors vector artwork, lifts raised surfaces, and rebalances layered shadows"; on for all sites or site by site; "Developers can also add a tag that causes Safari to ignore Adaptive Dark Mode"; iOS 27.3, macOS 27.3 and iPadOS 27.3 "will launch this December after several rounds of beta testing"; accessed November 4, 2026.
  8. 9to5Mac, “macOS 27.3 Golden Gate beta 1 now available, here's what to expect - 9to5Mac” accessed Nov 4, 2026Marcus Mendes, November 3, 2026, 10:10 a.m. Pacific: build 26C5035r; the install steps, back up the Mac, open System Settings, General, Software Update, click the i beside Beta Updates, choose macOS Developer Golden Gate Beta, click Done; accessed November 4, 2026.
  9. 9to5Mac, “macOS 27.3 adds native Markdown rendering to Quick Look - 9to5Mac” accessed Nov 4, 2026Marcus Mendes, November 3, 2026, 3:11 p.m. Pacific: as of macOS 27.3 beta 1, .md files are rendered with proper formatting in Quick Look rather than displaying their raw Markdown syntax; Apple's Siri AI skills use Markdown-based SKILL.md files; accessed November 4, 2026.
  10. 9to5Mac, “First macOS Sequoia 15.8.3 and macOS Tahoe 26.7.3 RCs now available - 9to5Mac” accessed Nov 4, 2026Marcus Mendes, November 3, 2026, 10:29 a.m. Pacific: the first release candidates of macOS Sequoia 15.8.3 (build 24H314) and macOS Tahoe 26.7.3 (build 25G416); their notes say only "security fixes for your Mac"; macOS Sequoia 15.8.2 and macOS Tahoe 26.7.2 were released the week before, on October 26, 2026; accessed November 4, 2026.

Then and now

All pairings

In 1997 we asked whether Mac OS 8.1 would slip to January with QuickTime 3.0; in 2026 the first beta of macOS 27.3 arrived November 3, a week after 27.2, with its features in Apple's notes on day one.

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