[jillesvangurp]

If you are serving customers and don't want to walk away from sizable percentages of your user base, you test on Chrome, Firefox, IE, Edge (old and the chromium one), Safari, Brave, and maybe even good old Opera. And if you are any good, it would be relatively rare for you to find issues.

IE is probably the hardest of these. The reason that still has non trivial amounts of users is because so many websites can't afford to lose access to those users.