https://www.techradar.com/news/safari-12-is-now-available-for-macos-sierra-and-high-sierra

Safari 12, the latest version of Apple’s web browser, is now available to download for people running macOS Sierra (10.12.6) and macOS High Sierra (10.13.6).

This new version of the browser, which is the default browser in the upcoming macOS Mojave (10.14) update, comes with a range of new features, and if you can’t wait until Mojave launches on September 24 you can get the updated browser for the older versions of macOS.