If the following webpage is correct about the
definition of a comment, HTMLParser.HTMLParser reports
valid (albiet strange) comments as being erroenous:

http://www.htmlhelp.com/reference/wilbur/misc/comment.html

This site gives '<!>' as an example of a valid html
comment. See attachment for what happens at the
console. A similar thing happens with other
(pathalogical) form of comments.