Quote from HTML of message page:
mhonarc was swift enough to capture this reference from an email header.
Yet the thread index did not respect this relationship. What is the
status of using "References: " references in threading?
A link to the referenced message will only exist if the referenced
message is in the same archive as the referer. In the archives that
Achim maintains, each month is a separate archive. Therefore, the
referenced message is probably in a previous month, or the referenced
message never was sent to the mhonarc list.
Comments should be made SGML-squeaky-clean. That means there is whitespace
after the <!-- and whitespace before the -->.
Well, I hate to burst your bubble, but the comments are legal.
According to ISO 8879:
10.3 Comment Declaration
 comment declaration = mdo, (comment, (s | comment)*)?, mdc
 comment = com, SGML character*, com
I suggest you do some reading about SGML.
P.S. The majority of my work involves SGML. And one of my Perl tools
is a DTD parser. I will concede that on a element validation side,
pages generated by MHonArc may not validate. However, that can be
controlled via the various page layout resources.