This bookmarklet reveal all the alt attributes of the <img /> tags (current page only)

alt-checker.js

javascript: (function() {
  function getTag(tag) {
    return document.getElementsByTagName(tag);
  }

  function a(o, a) {
    return o.getAttribute(a);
  }

  var images = getTag('img');

  if (images.length <= 0) return;

  var wrap = document.createElement('div');
  var wrapCSS = 'padding:5px;position:absolute;top:0;left:0;background:#fff;border:solid #ccc 1px;z-index:999;';
  var tblCSS = ' style=\'border-collapse:collapse;background:#fff;\'';
  var tdlCSS = ' style=\'padding:4px;border-bottom:solid #ffffff 2px;text-align:right;\'';
  var tdrCSS = ' style=\'padding:4px;border-bottom:solid #ffffff 2px;text-align:left;\'';

  wrap.style.cssText = wrapCSS;
  var content = '<table' + tblCSS + '>';

  for (var index = 0; index < images.length; index++) {
    content += (index % 2 == 0) ? '<tr bgcolor=#f6f6f6>' : '<tr bgcolor=%23f2f2f2>';
    content += '<td' + tdlCSS + '><img src=' + a(images[index], 'src') + '></td><td' + tdrCSS + '>' + a(images[index], 'alt') + '</td></tr>';
  }
  content += '</table>';
  
  getTag('body')[0].appendChild(wrap);
  wrap.innerHTML = content;
  wrap.onclick = function() {
    this.parentNode.removeChild(this);
  };
  window.scrollTo(0, 0);
})()

usage.md

Usage

1. Open alt-checker.js file as "RAW"
2. Select all the code
3. Drag & Drop it in your bookmarks