How much should I spend fixing Safari cross-browser issues?
In terms of present market share, Safari is the dominant browser for both macOS and iOS devices. Besides on iOS even Chrome and Firefox use Safari’s rendering engine. If you don’t mind writing off all iOS customers and most Mac devices users, then by all means don’t worry about Safari browser.
Do note, however, that Safari scores very high on HTML5 compatibility checking, so if your code is valid, it should work.