JavaScript 读取 iframe marginHeight 与 marginWidth 属性

<!DOCTYPE html>
<html>
<body>
<iframe id="myframe" src="demo_iframe.htm" 
marginheight="50" marginwidth="50">
<p>Your browser does not support iframes.</p>
</iframe>
<br><br>
<p>The value of the marginheight attribute and marginwidth are:
<script>
document.write(document.getElementById("myframe").marginHeight);
document.write(document.getElementById("myframe").marginWidth);
</script>
<p>
</body>
</html>