If you navigate to the Chapter 1 page on lpn.swi-prolog.org, then click on 1.1 Some Simple Examples, the page is blank.
If you visit the same link on the learnprolognow.org version of "Learn Prolog Now!" without SWISH it has content: http://www.learnprolognow.org/lpnpage.php?pagetype=html&pageid=lpn-htmlse1