Return false to prevent jumping scrollbars
Another common do-nothing insert to use for the href property is a pound sign (#), and while that will work, it'll also move the user’s scroll bar to the very top of your website, which can get quite annoying.
The way to get around the scrolling problem is to include a "return false;" in your event handler:
<a href="#" onmousedown="toggleDiv('mydiv'); return false;"> Toggle Div Visibility </a>
Again, basic stuff, easily overlooked.
(I wrote to Harry about this but I don't think he approved my comment. That's okay; it's why we have Trackback :)