Safari Technology Preview release 58 includes the same WebKit revisions that were present in Safari Technology Preview 57, but with Safari and WebKit features that will be coming to Safari 12 when it is released later this year.
Icons in tabs, improved Intelligent Tracking Prevention, and Automatic Strong Password functionality for automatically generating unique, strong passwords when signing up for accounts or changing passwords on websites.
Today's Safari Technology Preview update is available for both macOS High Sierra and macOS Mojave, the newest version of the Mac operating system that's currently being beta tested by developers.
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.