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HTML5 allows <img> elements to be associated with a "caption" by various means, in which case the alt attribute may be omitted, and AFAICT there's only one case when the alt attribute may be omitted even without a caption, which is: "4.8.2.1.11 An image in an e-mail or document intended for a specific person who is known to be able to view images" ...which is probably not something that you would see on the Web and so a validator could ignore that case, making alt required (when there's no caption), meaning that a validator would flag an error when there's no alt and no caption. From discussions on w3c-wai-pf it appears that it is not obvious from the spec that this is the case so it would perhaps be useful if the spec was clearer on this point.
I added a section for conformance checkers... not sure how else to do it.
Please refer to the Change Proposal "Replace img guidance for conformance checkers with suggested text" [1] for details on how to modify the HTML5 Specification so that automatic validators can programmatically detect the presence or absence of text alternatives on the img element. Thank you. [1] http://esw.w3.org/topic/HTML/ChangeProposals/ImgElement20090126 This is associated with HTML TRACKER ISSUE-31 http://www.w3.org/html/wg/tracker/issues/31 The full issue and is detailed at: Omitting Short Text Alternatives on <img> http://esw.w3.org/topic/HTML/IssueAltAttribute
The HTML Accessibility Task Force intends to track these issues, per the proposal at http://lists.w3.org/Archives/Public/public-html-a11y/2010Jan/0245.html.