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The alt tag is not supposed to be displayed as a tooltip. This behavior was introduced by Netscape 4, which then was emulated by MSIE. It is outside the spec.
http://www.mozilla.org/docs/web-developer/faq.html#alttooltip
Why doesn’t Mozilla display my alt tooltips?
Contrary to a popular belief stemming from the behavior of a couple browsers running on the Windows platform, alt isn’t an abbreviation for ‘tooltip’ but for ‘alternative’. The value of the alt attribute is a textual replacement for the image and is displayed when the image isn’t.
Mozilla doesn’t display the alt attribute as a tooltip, because it has been observed that doing so encourages authors to misuse the attribute.
* When the alternative text is shown in a tooltip, some authors write bad alt texts, because they intend the text as auxiliary tooltip text and not as a replacement for the image. (‘Bad’ in the sense that the textual alternative is less useful for people who don’t see the image.)
* When the alternative text is shown in a tooltip, other authors don’t want to supply textual alternatives at all, because they don’t want tooltips to appear. (Again, making things harder for people who don’t see the image.)
There is another attribute that Mozilla shows as a tooltip: title. In fact, the HTML 4.01 specification suggests that the title attribute may be displayed as a tooltip. However, this particular display method is not required and some other browsers show the title attribute in the browser status bar, for example.
At this point some people feel compelled to post a “But IE…” rant in the newsgroups or in Bugzilla. Please note that Mac IE 5 behaves in the same way as Mozilla when it comes to the alt and title attributes. Windows IE also shows the title attribute in a tooltip.