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chromium/content/test/data/accessibility/html/img-link-empty-alt.html 

<html>
  <body>
    <a href="foo">
      <img src="unread.jpg" alt="" role="presentation">unread
    </a>
    <a href="foo">
      <img src="read.jpg" alt="">read
    </a>
    <a href="foo">
      <img src="read.jpg" alt>read
    </a>
    <a href="foo">
      <img src="read.jpg">read
    </a>
  </body>
</html>