<!DOCTYPE html>
<html>
<body>
<textarea rows="4" cols="50">Click me (or use the tab key) to assure that I get focus. Then, click the input field to make sure that it gets focus.</textarea><br><br>
Input field: <input type="text" onfocus="getRelatedElement(event)">
<p><strong>Note:</strong> The relatedTarget property of the FocusEvent object is not supported in IE8 and earlier. </p>
<script>
function getRelatedElement(event) {
alert("The related element that lost focus was: " + event.relatedTarget.tagName);
}
</script>
</body>
</html>