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Apple’s goal with Safari Technology Preview is to gather feedback from developers and users on its browser development ahead of launch. Safari Technology Preview can be run side-by-side with the existing Safari browser and while aimed at developers, it does not require a developer account to download.
If you already have an older version of Safari Technology Preview, the latest version is available through the Mac App Store’s Updates tab. If you want to take Safari Technology Preview for a spin, go ahead as the installer does not require a developer account to download.
Release notes are available on Apple’s Safari Technology Preview website.