LayoutTests/fast/html/tabindex-removal.html

<div id="a" tabindex="1">PASS</div>

<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
    }
    var a = document.getElementById('a');
    a.removeAttribute('tabindex');
    a.addEventListener('focus', function (e) {
        a.innerText = 'FAIL';
    }, false);
    a.focus();
</script>