This nice little code will cause the browser to literally bounce off the walls, shaking all over the desktop. The key here is timing; if your coworker's office cubicle is abandoned, you should have a pretty good idea of when she'll be coming back, and adjust the code as needed. First, try out the code below by pasting it into the browser:
As you can see, the hyper browser is very amusing, but wouldn't last long enough for a sneak-attack prank. For this escapade, you'll likely need to edit the code to last a little longer. To do so, find the statement that says i = 35, and change the number to something larger (300 will buy you about 30 seconds, 600 a minute, and so on).