The css and js files are commented out in the header yet are still imported into the head by Sitebuilder (see raw code). The alert that pops up reports the loaded files jQuery UI version. If you cut the css and js out of the head and update the page you get the expected result.
This is a Tabs demo straight from the jQuery UI website: https://api.jqueryui.com/tabs/#entry-examples
If you edit the code and remove the comment from the front of the tabs line you get some interesting errors (line 42).