This file: https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/master/repo/src/main/resources/index.html is using very old xslt APIS. Here a more modern example, we could use as a starting point https://developer.mozilla.org/en-US/docs/Web/XSLT/XSLT_JS_interface_in_Gecko/Basic_Example