WebView browser = new WebView();
WebEngine webEngine = browser.getEngine();
ClassLoader classLoader = getClass().getClassLoader();
String s = classLoader.getResource("subfolder/abc.html").toExternalForm();
webEngine.load(s);

file:jar:C:/Something/MainA/libs/SecondB.jar!/subfolder/abc.html

String fileloc = "/docs/help/userguide.html";
...
String fullLink = getClass().getResource(fileloc).toExternalForm();
...