This CSS method of doing a rollover causes a flicker the first time the mouse is over the link (as long as the image has not been cached). Using 2 images (one for the normal state and one for the hover state) causes the browser to only download the first one until the mouse is over the link, causing a short bit of time where there is no background image.
The flicker is short if you have a high speed connection, but on a modem, it can seem like an eternity...
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