ergophobe - 8:55 pm on Apr 26, 2012 (gmt 0)
Just me and this is not advice: I never use a standalone keyword as alt text unless it describes the image perfectly. For example, let's say I had a PNG/GIF chart of widget futures. I would probably choose "chart of widget futures, 2005-2012" as the alt text rather than "widget futures".
90% Accessibility. "Widget futures" doesn't tell a blind user anything. If the chart is already illustrating a page on widget futures, she knows that already. What she needs to know is whether this is a filler image just for color or real data and, if it's real data, maybe she needs to get someone to pull some numbers off it for her.
10% Searchability. I'm not going to rank for widget futures, but I just might rank for "chart of widget futures".
My rule of thumb is not try to include content that doesn't offer something to the visitor. That doesn't mean I'm not verbose, rambling and often horribly off-topic and frequently spouting useless noise. It means that when I do so, it's by accident, not on purpose ;-)