Problem: OpenJDK 1.8 builds don't include JavaFX libraries.
Workaround: (Assuming the platform is GNU/Linux.)
Copy the JavaFX runtime JAR and other related files included in a ZuluFX build (https://www.azul.com/downloads/zulu/zulufx/) to the following locations in the home directory of the OpenJDK 1.8 build:
cp $ZULUFX_HOME/lib/javafx.properties $OPENJDK_HOME/lib/
cp $ZULUFX_HOME/lib/amd64/libprism_* $OPENJDK_HOME/lib/amd64/
cp $ZULUFX_HOME/lib/amd64/libglass.so $OPENJDK_HOME/lib/amd64/
cp $ZULUFX_HOME/lib/amd64/libjavafx_* $OPENJDK_HOME/lib/amd64/
cp $ZULUFX_HOME/lib/ext/jfxrt.jar $OPENJDK_HOME/lib/ext/
And ensure the GTK+ 2.18 or higher is also installed.