Debug tabindex with javascript by listening "tab" key event and show in console the active element. Tested in Chrome 25 and Firefox 19

tabindexDebug.js

(function(){
    "use strict";
    document.addEventListener('keydown', function (event) {    
        setTimeout(function () {
            var key, win, frameEl, activeEl, uri, msg;
            msg = "";
            key = window.event ? event.keyCode : event.which;
    
            //Tab
            if (key === 9) {
    
                activeEl = document.activeElement;
                while (activeEl && activeEl.tagName === 'IFRAME') {
                    activeEl = activeEl.contentWindow.document.activeElement;
                }
                if (activeEl) {
                    msg += activeEl.id + ' - ' + activeEl.tagName;
                    win = activeEl.ownerDocument.defaultView;
                    uri = win.location.href;
                    msg += ' - ' + uri;
                    msg += ' (tabindex=' + activeEl.getAttribute('tabindex') + ')';
                }
    
                frameEl = win.frameElement;
                if (frameEl) {
                    msg += 'in frame ' + frameEl.id + ' (tabindex=' + frameEl.getAttribute('tabindex') + ')';
                }
                console.log(msg);
                console.log(activeEl, frameEl);
                activeEl = null;
            }
        }, 500);
    });
})();