I got it now. Specifically, the comments I have downvoted will have the "class" attribute set to "down mod" on the anchor text "Vote down" (which causes the anchor text to be bolded) whereas the comments I have not downvoted will have it set to "down".
Exactly!
Downvote buttons seem to have just stopped working for me, saying I need at least 1 karma. I'm posting in case it leads someone to fix the problem.