The web app we develop has occasional issues with Safari that we need to look into. Things not working in both Firefox and Chrome is extremely rare for us and usually fixed pretty easily in the rare case we notice it.

FWIW, that mirrors my experience entirely. i primarily develop with FF and test with Chrome/Chromium as a sanity check. Safari, once the app is out the wild, is always the outlier (and i don't use it, so can't fix its shortcomings except via trial and error).