Safari Technology Preview release 38 includes fixes and improvements for Fetch API, Web Payments, CSS, Web API, Media, Apple Pay, Web Inspector, and WebDriver. Today's update also enables the Beacon API by default and implements new Beacon API features.
With Safari 11 now available to developers through the macOS High Sierra beta, Apple is providing two versions of Safari Technology Preview, one for macOS Sierra users and one for those using macOS High Sierra.
The Safari Technology Preview update is available through the Software Update mechanism in the Mac App Store to anyone who has downloaded the browser. Full release notes for the update are available on the Safari Technology Preview website.
Apple's aim with Safari Technology Preview is to gather feedback from developers and users on its browser development process. Safari Technology Preview can run side-by-side with the existing Safari browser and while designed for developers, it does not require a developer account to download.