It is hoped to use modern JS libs for all of our new UI work (create item, 1st run plugin bundling experience etc). Jenkins Core currently uses old JS libs, mainly prototypejs. This library has been dead for years + we are using a forked version of it. We were hoping to get away from it in the newer work.
We are also currently using a forked version of HtmlUnit in Jenkins Core. It's an old version (2.6 - latest is 2.18). This version of HtmlUnit doesn't support use of jQuery (and probably lots of other JS libs we would want to use) e.g. it complains about how the "isArray" method is defined in the lib. They claim that the latest version of HtmlUnit "fully supports" jQuery (I'm not convinced).
The problem for us is that the Jenkins Core test harness relies heavily on HtmlUnit. There are many many tests written to run against the forked version, which means they fail when we try to do anything that that version of HtmlUnit doesn't like e.g. introduce jQuery.