I installed Jetty 7 and configured it correcty for IdP Shibboleth following this guide: https://wiki.shibboleth.net/confluence/display/SHIB2/IdPJetty7Prepare
<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).
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.
Add this in your file, to resolve the dependency:
<dependency>
<groupId>xml-apis</groupId>
<artifactId>xml-apis</artifactId>
<version>1.4.01</version>
</dependency>