Both Java Web Start and Java Plugin have code to compare the requires jnlp specification version with the ones they support.
In Java Web Start, this is in Launcher.java, and allows specifications that include:
1.0, 1.5, 6.0, 6.0.10, and 6.0.18.
In Java Plugin, there is the same method, JNLP2Manager.isValidSpecificationVersion, but it only claims to support 1.0, 1.5, and 6.0.
needs to be made up to date with latest JNLP spec versions as they come out