chromium/third_party/blink/web_tests/images/update-alt-text.html

<!doctype HTML>
Ensure we can update the alt text on an image that has failed to load.
<img id="img" src="none" alt="FAIL">
<script>
    document.body.offsetTop;
    var img = document.getElementById('img');
    img.onerror = function() { img.alt = "PASS"; }
</script>