lucy24 - 9:54 pm on Aug 22, 2011 (gmt 0)
The "longdesc" is not recognized by all browsers, so don't rely on it for essential information. It's a link to longer text, not a kind of mega-alt. w3c says
The longdesc attribute is so poorly supported that it should not be used.
A lot of the tutorial is out of date but this bit is even more true today, since "longdesc" is not in html5 at all-- probably precisely because nobody was using it correctly.
"alt" is required by the validator (and by human decency, unless it's just a decoration). Any user who has images turned off for any reason, or who uses a text-reading device, will get their only information from the "alt".
"title" and "alt" should be mutually exclusive. The alt displays when the picture doesn't; the title appears when the picture is there. At one time w3c even had a blurb explaining that the reason Firefox was so odd in it treatment of alt/title is that it was the only browser that did what it was supposed to do. (Haven't checked, but I really hope this is no longer true.)