I was in college when XHTML was all the rage and everything we wrote had to pass validation. I still get uncomfortable adding breaks without closing them.
There used to be a commenter here who would end all his comments with a closing paren, even though there had not been an opening paren. It led to a surprising number of flamewars! )
Edit: hmm, I couldn't really find any flamewars, but it did lead to objections:
Hilarious. To fix this, one would have needed to put an open parenthesis in a sibling comment and hope to get upvoted more than the comment with the closing parent.