I see that everytime, mostly in Wordpress / Drupal / Whatever templates :

<ol class="commentlist">
...
</ol><!-- .commentlist -->

Don't ! here is why :

• No one but an hypothetic future developer will ever need this. But you serve it to the entire world.
• It makes HTML file heavier, since the browser will download these comments. At least use PHP comments.
• With correct indentation, this is most of the times useless. Code is readable enough if correctly indented. It is actually less readable and adds noise.