Previously, VM failed in case of improperly option and -XX:+IgnoreUnrecognizedVMOptions flag passed as options.

Now -XX:+IgnoreUnrecognizedVMOptions hides improperly specified VM options. Example:

JDK 9 b66:

java -XX:MinTLABSize=0
MinTLABSize of 0 is invalid; must be at least 1
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit

The same message with -XX:+IgnoreUnrecognizedVMOptions.

New JDK (nightly build):

The same behavior (VM exits with error message) as in previously promoted JDK with new message:
java -XX:-IgnoreUnrecognizedVMOptions -XX:MinTLABSize=0 -version
size_t MinTLABSize=0 is outside the allowed range [ 1 ... 4294967295 ]
Improperly specified VM option 'MinTLABSize=0'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

JDK continues to work with out-of-range message:
java -XX:+IgnoreUnrecognizedVMOptions -XX:MinTLABSize=0 -version
size_t MinTLABSize=0 is outside the allowed range [ 1 ... 4294967295 ]
java version "1.9.0-internal-fastdebug"
Java(TM) SE Runtime Environment (build 1.9.0-internal-fastdebug-20150624183527.jesper.main2rt-b00)
Java HotSpot(TM) Server VM (build 1.9.0-internal-fastdebug-20150624183527.jesper.main2rt-b00, mixed mode)