This is a demonstration of an XHTML document (served as text/html) using a comment within the script element that is designed to hide scripts from older HTML user agents. See HTML comments in scripts for more information

Because the document is parsed with HTML rules, rather than XML rules, the script element is parsed as CDATA and is therefore treated as though it is not commented out. Therefore, the script does work.

If the script executed, you would have seen an alert containing "Hello World!".