Below are two messages, which I stole out of a mailing list and then
hacked on manually in a different personal group until I got to a
point where one difference between them determines whether they parse
correctly as HTML or not.

The first one displays as a bunch of raw (but oddly quote-colorized :-)
HTML directives.  The second one displays as properly parsed and
formatted text.  The difference between them is the presence or
absence of two lines, which looks to me like rather uneventful text.

In fact, one can add or delete nearly any single <div></div> segment
to or from these messages, to toggle success of HTML parsing and
display.  I do not understand this at all.

I am almost surely looking at a W3 error, but since I found it within
Gnus, I figured I'd ask here first.