The version of the JDK used by the browser must be greater than or equal
to the version you use to compile. Thus, if you compile  with JDK 1.5,
but your browser uses JDK 1.4.2, you will have compatibility issues.
Make sure your compiler version is lower than or equal to your browser
version. You should also advertise this to your users: "This applet
requires JDK 1.5 or above."