This `</div>` ending tag closes the sidebar. It's a good idea to comment here so that when the document gets so long that opening and closing tags are no longer able to be on the screen at the same time, you can still know what opening tag it belongs to.

If the opening tag is `<div id="sidebar">` the closing tag would have a comment like so `</div><!-- #sidebar -->`

Do this for all major elements with IDs