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Excellent article Derek.
Like Patrick mentioned, it all comes back to WAI checkpoint 6.1 and Section 508, Part 1194.22, Paragraph d. Make sure that pages are readable and usable without style sheets.
I’ve taken the approach that there are three levels of non-text content on a site:
1. Eye-candy: Things that serve no purpose other than to make a site visually appealing/attractive and (in many cases) satisfy the marketing departments. There is no content value (though there may be value to a sighted user).
2. Mood-Setting: This is the middle layer of graphics which may serve to set the mood or set the stage as it were. These graphics are not direct content and may not be considered essential, but they are important in that they help frame what is going on.
3. Content/Function: This includes the actual navigation as well as actual content.
Given those three, I never alt-ify the first group unless there is something there which will enhance the usability of the site for someone using a non-visual user agent.
I try to alt-ify the second group as makes sense and is relevant. There may be times when doing so may be annoying or detrimental to other users and I’ll avoid it. For instance alt text that is identical to an adjacent text is unnecessary, and an irritant to screen reader users. I recommend a CSS background image or alt=”” in such cases. But sometimes, it’s important to get this content in there for all users.
I always alt-ify the third group.
What’s necessary and what’s not depends a lot on the function of the image and its context on the page. The same image may require alt text (or title or longdesc) in one spot, but not in another. If an image provides absolutely no content or functional information, CSS background images or alt=”” may be appropriate to use. But if the image provides content or adds functional information an alt would be required and maybe even a long description would be in order.
In many cases this type of thing is a judgement call. Use your best :-)