Limitations

The code above requires JavaScript 1.1 and above to execute. This means that if your visitor is using a browser prior to Netscape 3.0 or Internet Explorer 4.0, they will not be able to view your rollovers. 

The links, however, will still work correctly when clicked (even on browsers with JavaScript disabled or with no JavaScript support at all). This is important since mouseover effects are purely cosmetic, and you don't want them to detract from the overall functionality of your site. 

Note that you should probably also make sure that the files you use for the buttons are small. If your files are too big, the image rollovers (or mouse-overs) will not have the smooth effect of buttons popping up and down that you intend.