In addition to the UA spoofing others mention, there's also forward conpatibility. If your app holds a mini "caniuse.com" UA-feature mapping, you have to keep

In addition to the UA spoofing others mention, there's also forward conpatibility. If your app holds a mini "caniuse.com" UA-feature mapping, you have to keep it up to date infinitely forward in time, or new browsers won't work (think Brave/etc) Better to detect features directly, and reward anyone who implements them, rather than just hardcode the present incumbents.