gmiller - 12:28 pm on Dec 21, 2000 (gmt 0)

ALT attributes (they're not actually tags) are mandatory starting in HTML 4.0. alt="" is fine, though, if that's appropriate.

ALT attributes are displayed when using browsers that don't support graphics (lynx, w3m, etc), when the user has graphics turned off or clicks stop before the image finishes loading, and I believe older Netscape versions displayed them while the image was loading.

On a related note, there was a lawsuit filed over the Olympic web site's reliance on graphics and failure to use alt attributes where appropriate. I believe it's still pending.