Dave Winer: “The XML icon on Scripting News has a tool-tip that’s visible when you hover your mouse over it. It says ‘Click here to see an XML representation of the content of this weblog.’”
Well, no, it doesn’t. It does only on Internet Explorer, hardly a standards-compliant browser. If you want a tooltip to appear on any sane browser (including IE6, I think), put the tooltip text as the value of the title attribute of the link. The alt attribute of img is for providing an alternative description of the image, not of the destination of the link.
Of course, the mozilla-suite’s implementation of ‘title’ tooltips is horrendously broken, so it’s no suprise people may avoid using it. See https://bugzilla.mozilla.org/show_bug.cgi?id=218223