After Microsoft giving up on their rendering engine and joining the Chromium train I think the internet is in a perilous place. We're down to Blink, Gecko, and WebKit. Apple's monopoly on iOS browsers (soon to be undone) is unfortunately one of the last things keeping us from recreating the dark ages of IE's near monopoly on the market. I'd recommend at least supporting a non-Blink based browser like:

  * Lunascape (actually has all 3 major engines built in. Great for web dev)
  * Pale Moon - uses Goanna, a fork of Gecko
  * Waterfox - focused on speed and privacy and compatible with Firefox addons

See also: Floorp, IceCat, and SeaMonkey