I remember running Java applets under Netscape 3.x and 4.x on System 7.5 on a 200Mhz PPC 603ev and 16MB RAM. It was “fine” mostly, but loading was slow as mud (though that might’ve just been the 28k dialup) and they crashed Netscape or the whole system a lot more than the rest of the web did. Technically usable, but practicality was questionable.