On every page of moodle there is a #maincontent in the HTML. Normally, you might expect this to be a div that wraps around the main content. But in Moodle it is an empty <span> that is set to {{display: block;height: 1px;overflow: hidden;} so it's a usually invisible line at the top of each page's content.

The only use I've found (while looking at MDL-34687) is an over-elaborate way of setting the width of embedded objects. It's possible this is some old hack to fix IE6, but can probably be removed now.