The closing li tags were optional in HTML, up through middle-to-late 1990s. This is no longer true for any even remotely recent version of HTML. The last several versions of HTML are XML document types, so all tags must be closed.

That is not correct. HTML and XHTML are not the same, and every version of HTML is an application of SGML. HTML thus allows for markup minimization when the corresponding document-type definition (DTD) says so, and all of the HTML DTDs – going back to the original version – define the LI element's end tag to be optional.

As long as Perl Monks is served up in HTML, it will be perfectly acceptable to omit any tags the corresponding DTD says are optional.

The W3C's HTML and XHTML Frequently Answered Questions addresses the underlying issues in detail.

Cheers,
Tom