Well you don't need to test on lots of versions, since updates are usually automatic now (or at least highly encouraged on startup). But you definitely need to

Well you don't need to test on lots of versions, since updates are usually automatic now (or at least highly encouraged on startup). But you definitely need to test on Chrome and Firefox in the browser (maybe also Safari?), and both Chrome and Safari on mobile, since there are JavaScript APIs and CSS features not supported by all of them. The latter is especially true when you're trying to make a PWA or 'web app', since Apple does not like letting websites have similar abilities to native apps on their devices...