I\'ve been asked to make a sponsored background (site takeover) for one of our sites and the question arose of whether I can make the logos in the tiled background clickable
You'd need to be careful around event order (event capturing vs. bubbling) differences between IE and Mozilla. If you have an element with an onClick event that essentially takes up the entire page, and then other clickable elements on "top" of it, clicking on one of those elements can cause BOTH events to fire, which is likely not the intended functionality.
PPK explains it better than I can.