I think they use the title tag. I've been wondering about this myself but never bothered to investigate.
I don;t fully understand your question ? You mean an image rollover ? Or just want the ALT text to show up when you move the mouse over the image ?
The alt attribute contains (see http://www.w3.org/TR/html4/struct/objects.html#adef-alt ) alternative text to the image. That a tooltip is shown on mouseover/hover is considered an IE bug. The proper attribute to use is title. In fact, you may kill the alt tooltip by including the title attribute. If you want no tooltip at all, give the title null content, thus <img src="aster.jpg" alt="a blooming aster" title="">. Authoratative comments may be found at http://annevankesteren.nl/2004/12/alt-attribute http://annevankesteren.nl/2006/02/alt http://annevankesteren.nl/2006/02/tooltips (sorry, not allowed to create links yet—a little help from the mods?) cheers, gary