// ==UserScript==
// @name	Alt Tooltip
// @namespace	http://www-ui.is.s.u-tokyo.ac.jp/~kobayash/
// @description	Makes any ALT text pop up.
// @include	*
// ==/UserScript==

(function () {
  const elements = document.evaluate('/descendant::*[@alt and not(@title)]', document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
  
  for (var i = 0; i < elements.snapshotLength; ++i) {
    var o = elements.snapshotItem(i);

    o.title = 'ALT : ' + o.alt;
  }
})();