index.html
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
</head>
<body>
<h1>
<iframe src="./sub.html" frameborder="0"></iframe>
</h1>
</body>
</html>
sub.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
</head>
<body>
<button onclick="clickHandler()">click</button>
<script>
function clickHandler() {
alert(window.frameElement.parentElement.nodeName); // H1
}
</script>
</body>
</html>