This is a demonstration of an XHTML document (served as application/xhtml+xml) 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 XML rules, rather than HTML rules, the script element is parsed as #PCDATA and is therefore commented out. Therefore, the script will not work.

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