A DESCRIPTION OF THE PROBLEM :
The JNLP files have been incorrectly updated. A copyright comment section <!-- ... --> has been placed *before* the <?xml declaration! This is not valid XML syntax; comments cannot appear before the XML declaration.

Either the XML declaration needs to be:
(a) moved to the beginning of the files (above the copyright comment)
(b) removed completely (still valid XML)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Clicking on the orange-ish 'Launch' buttons within the 'Try this:' sections launches a swing demo
ACTUAL -
Clicking on the orange-ish 'Launch' buttons within the 'Try this:' sections attempts to launch a swing demo. As the jnlp files are invalid XML, an error dialog appears.

URL OF FAULTY DOCUMENTATION :
https://docs.oracle.com/javase/tutorial/uiswing