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Spec says: ]] When an a element that creates a hyperlink, or a button element, has no textual content but contains one or more images, the alt attributes must contain text that together convey the purpose of the link or button.[[ Therefore one would expect that the following examples would trigger errors: <a href=foo ><img src=bar alt=''" ></a> <a href=foo ><object data=bar type="image/gif"></object></a> <button type=button ><img src=bar alt="" ></button> <button type=button ><object data=bar type="image/gif" ></object></button> But currently, as the validator considers the above examples as valid. Please fix. See message to public-html/public-html-a11y: http://lists.w3.org/Archives/Public/public-html/2012Feb/0407