title: "An img element must have an alt attribute, except under certain conditions." What conditions?
body: <p>This is what the W3C Validator prints for an img tag without an alt attribute:</p>
  
  <p><em>"An img element must have an alt attribute, <strong>except under certain conditions</strong>."</em></p>
  
  <p>I can't find anything about it... at least nothing more than what the alt tag is about.</p>
  
  <p>Does anybody know <strong>what conditions</strong>?</p>