Run your HTML through the W3C validator because before looking for bugs, the first step must always be "find your typos".
After you've corrected the HTML, see who still breaks and post again.
As a side note: you have a div wrapping your menu, but you don't need it. You're using :hover to do everything, but that means it won't work for keyboard users. And the display: none/block method is known to keep menus hidden from folks using screen readers.
They get around the problem of display: none/block, so their menu is fairly accessible.
You can make it keyboard accessible too, so I guess that's my other gripe with HTMLdog is they don't build in keyboard accessiblity from the beginning and they could have.