Apple has begun testing Safari 10 with developers running the 2014 and 2015 editions of macOS, preparing for a fall release of the updated browser to users of Yosemite and El Capitan.
Safari 10 was introduced earlier this month as part of macOS Sierra, this year's operating system upgrade.
Apple typically supports its newest browser on three editions of macOS: The latest version and its two predecessors. The now-current Safari 9, for example, receives updates, including security patches, on last year's El Capitan, 2014's Yosemite and 2013's Mavericks.
Safari 10 will be supported on Sierra, El Capitan and Yosemite. Meanwhile, Mavericks will remain on Safari 9.
The Safari 10 preview is currently available only to registered Apple developers, who pay $99 annually for access to early builds, development tools and documentation.
The general public will get its first look at Safari 10 next month after Apple opens up its broader-based public beta program for Sierra. Those who have signed on to the beta preview will also be able to download preliminary versions of Safari 10 for El Capitan and Yosemite, running the preview browser but sticking with their older, more stable operating systems.
Some of Safari 10's signature features will be available only within macOS Sierra, including web-based Apple Pay -- where payment is authorized with an iPhone or Apple Watch -- but others will be supported by older versions of the operating system. Among the most notable are the new ability for developers to distribute and sell Safari add-ons in the Mac App Store, and easy portability of iOS content blockers to macOS.
If Apple replicates last year's beta schedule, it will release the first public preview of macOS Sierra and Safari 10 around July 14.