Challenge: analyse how browsers enter US Airlines’ membership website

We recently investigated a bug report about this US Airways login form. Pressing enter to log in fails – but only in Opera. The question is why? Read on if you want to try to analyse the problem.. …

Colleague Kåre dig through the layers of form validation scripts and came up with this demo of the problem in all its complexity:

<form method="post" action="#" onsubmit="javascript:return original_onsubmit();">
	<input type="text" value="Press Enter">
</form>
<script type="text/javascript">
function original_onsubmit()
	{
	if((typeof(window.event) != "undefined") && (window.event != null))
		{
		window.event.returnValue = true;
		}
	return true;
	}

var f = document.forms[0];
var userName = document.getElementsByTagName('input')[0];
userName.focus();

if(document.addEventListener)
	{
	f.addEventListener('submit',foobar,false);
	userName.addEventListener('keypress',loginkeypress,false);
	}
else if(document.attachEvent)
	{
	f.attachEvent('submit',foobar);
	userName.attachEvent("onkeypress",loginkeypress);
	}

function loginkeypress(e)
	{
	e = e?e:window.event;
	if(e != null && e.keyCode == 13)
		{
		if(e.preventDefault)
			{
			e.preventDefault();
			}
		else if(window.event)
			{
			e.returnValue = false;
			}
		document.location = 'javascript:submitform()';
		return false;
		}
	}

function foobar()
	{
	alert('FAIL');
	}

function submitform()
	{
	if(f.onsubmit() != false)
		{
		f.submit();
		}
	}
</script>

Among the main four browser engines, Opera's is the only one to call foobar(). Can you tell why, and propose a fix?

If you need to see the code running, it's here:
CORE-32417.htm

Discussion open in the comments. I have an analysis and a list of 5 different ways we might fix it, which I'll post later – for now it will be interesting to see your proposals 🙂