For initial launch, the latest versions of Chrome, Firefox, Safari, and Edge, on Windows and Mac, Android, and iOS (or only desktop if your service doesn't make

For initial launch, the latest versions of Chrome, Firefox, Safari, and Edge, on Windows and Mac, Android, and iOS (or only desktop if your service doesn't make sense to use on mobile). Support or accept reduced functionality on the last Internet Explorer (a LOT of consumers are still on older Windows at home). Adjust this after launch after watching your traffic - you can drop support for browsers that provide little to no traffic; then add it back later when you have enough traffic to warrant it. Watch how many visitors you get from other browsers and older versions; add support when visitor volume * conversion rate / cost to support suggests higher ROI than other potential uses of that time. Finally, you may want to support more browsers on your sales page than the app itself. You want to be able to sell your app even to people who find you using Opera Mobile on Windows Phone; but if they want the service enough, you can probably persuade them to fire up Chrome or Firefox to actually run it.