NoClassDefFoundError: org/w3c/dom/ElementTraversal

It looks like ElementTraversal is part of xml-apis-2.10.0.jar which should have been provided with your Shibboleth installation.

So if you were following these instructions you should also have followed this step:

Endorse Xerces and Xalan by creating the directory JETTY_HOME/lib/endorsed/ and copy the .jar files included in the IdP source endorsed/ directory into the newly created directory.

So long as you copied all the jar files appropriately, it should be okay. My guess is that you missed out that step - or perhaps only copied the jar files with Xalan or Xerces in the name, and missed the other two.

In my case the problem was fixed by changing xml-apis version from 1.3.04 to 1.4.01.

BTW, version 2.10.0 doesn't exist in Maven Central (I'm curious where did Jon Skeet find it).