You could ask the question: How do I know, that using this applet is safe? How can I be sure that there is no Trojan horse in this program, sniffing to my password?
The answer is simple: you can't. You must trust the Administrator of this site. The source of this program is free, anybody can check through the whole program code. (If you have checked, you notice, that it is safe.) But even then, how can you be sure that the applet is compiled from exactly this code? It is also simple: if you don't trust us, compile it for yourself, and use your own version. That must be safe. Or you should just trust us, like you use the linux or windows written by someone else.