Mo' Betta Rollovers
http://www.alistapart.com/stories/rollovers/

Murtaugh, Tim
List Apart, A
2002

Abstract:

For years we’ve been preloading our on–state images to assist the browser in its rollover presentation. Preloading increases the weight of the initial download, but adds to usability by decreasing the wait time for an on–state image to appear. Decreases, but may not eliminate. Even when preloaded, some browsers present momentary pauses prior to rendering the image. Besides being annoying, this momentary pause can cause problems when a visitor mouses over an element in passing or too rapidly. If the mouse movement is too fast, the browser does not have time to render the image and, worst–case scenario, can present you with a broken image icon.