For example, I have a directory with *.css file in it (src) and I'm trying to convert this CSS to BSS:

dmitry.ginzburg@oracle-ginzburg:/local/work/packagerTestsSuite/apps/CssToBinTest$ javafxpackager -createbss -srcdir src -outdir dist
Error: jfxrt.jar needs to be on classpath for -createbss and for -createJar without -nocss2bin

There's no option to javafxpackager to set classpath in documentation, so if I try to use -classpath option I'll get the same result:

dmitry.ginzburg@oracle-ginzburg:/local/work/packagerTestsSuite/apps/CssToBinTest$ javafxpackager -createbss -classpath $JAVA_HOME/jre/lib/jfxrt.jar -srcdir src -outdir dist
Error: jfxrt.jar needs to be on classpath for -createbss and for -createJar without -nocss2bin
dmitry.ginzburg@oracle-ginzburg:/local/work/packagerTestsSuite/apps/CssToBinTest$ ls $JAVA_HOME/jre/lib/jfxrt.jar
/local//jdk/jre/lib/jfxrt.jar

Then I'm trying to extend the output by verbose option:

dmitry.ginzburg@oracle-ginzburg:/local/work/packagerTestsSuite/apps/CssToBinTest$ javafxpackager -createbss -verbose -srcdir src -outdir distException in thread "main" com.sun.javafx.tools.packager.PackagerException: Error: jfxrt.jar needs to be on classpath for -createbss and for -createJar without -nocss2bin
at com.sun.javafx.tools.packager.PackagerLib.loadClassFromRuntime(PackagerLib.java:1413)
at com.sun.javafx.tools.packager.PackagerLib.createBinaryCss(PackagerLib.java:1438)
at com.sun.javafx.tools.packager.PackagerLib.createBinaryCss(PackagerLib.java:1384)
at com.sun.javafx.tools.packager.PackagerLib.createBinaryCss(PackagerLib.java:1377)
at com.sun.javafx.tools.packager.PackagerLib.createBinaryCss(PackagerLib.java:1369)
at com.sun.javafx.tools.packager.PackagerLib.generateBSS(PackagerLib.java:487)
at com.sun.javafx.tools.packager.Main.main(Main.java:406)
Caused by: java.lang.ClassNotFoundException: com.sun.javafx.css.parser.Css2Bin
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at java.net.FactoryURLClassLoader.loadClass(URLClassLoader.java:789)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at com.sun.javafx.tools.packager.PackagerLib.loadClassFromRuntime(PackagerLib.java:1411)
... 6 more

So it tries to use some class com.sun.javafx.css.parser.Css2Bin:

It it really existing class:
dmitry.ginzburg@oracle-ginzburg:/local/work/packagerTestsSuite/apps/CssToBinTest$ unzip -l $JAVA_HOME/jre/lib/jfxrt.jar | grep -i Css2Bin
     2211 2012-12-10 17:28 com/sun/javafx/css/parser/Css2Bin.class

It is important bug because it blocks the whole javafxpackager task: createbss.