For as long as I can remember (I think going back to IE3), Internet Explorer has displayed a ToolTip containing the contents of the ALT attribute for IMG elements. Other browsers including Netscape 4 also showed the ToolTip. In IE7, this looks something like this:
In IE8 standards mode, the ToolTip is no longer shown. This is a change we made to be more standards compliant, more interoperable with other current browsers, and to encourage more accessible mark-up. ToolTips are still shown in quirks and IE7 standards modes.
The problem comes because having the ALT attribute displayed as a ToolTip encourages two behaviours that arenâ€™t compatible with the accessible web. The first is that content authors naturally enough set their ALT text to be something appropriate for a ToolTip but not ALTernate text to represent the image in scenarios where users canâ€™t see the image. The second is that content authors actively donâ€™t put alternate text because they donâ€™t want a ToolTip displayed. Neither of these is desirable.
The solution for content authors is to use the TITLE attribute if they want a ToolTip. The TITLE attribute applies to other elements with the same effect and not just images. The ALT attribute should be used for text thatâ€™s an alternative to the image.