Updates of Safari are made available for the two preceding releases of macOS. For example, Safari 13 shipped with Catalina, but it's also available for Mojave

Updates of Safari are made available for the two preceding releases of macOS. For example, Safari 13 shipped with Catalina, but it's also available for Mojave and High Sierra. Any OS features not available on those earlier OSes are disabled, of course, but Web features tend to be unaffected. For example, Safari 13 doesn't support dark mode on High Sierra.