Applets can read quite many of system properties by using:

String ss = System.getProperty(String key):

java.version
java.vendor
java.vendor.url
java.class.version
os.name
os.arch
os.version
file.separator
path.separator
line.separator

Applets are prevented from reading these system properties:

java.home
java.class.path
user.name
user.home
user.dir