This (modification of a very) ancient guy requires only Java 1.4.2 (so these days it should probably work on pretty much any browser that's got a real Java installed).
(Getting the 'input focus' into the applet, so that it sees your keystrokes, sometimes seems to require moving the mouse outside the browser window, and then back. Not sure if that's an issue anymore.)
If you can make level 7 that's pretty good. Making level 8 is pretty insane.