Short answer is don't use QuickBaseClient.js - it is archaic in many ways and isn't really even supported by QuickBase.
An answer from 2004 states:
Internet Explorer is the only browser that now doesn't support XPathEvaluator. You can use selectNode() and selectSingleNode() on the DOM Document object instead (IE only).
All of these constructs are outdated even if the browsers may support them.
For what is it worth I can log XPathValidator in the console of Chrome, Firefox and Edge but I can't test on IE browsers. So try Edge instead of IE if you are on a Microsoft box.
