[Old Pedant]

As an alternative:

<form method="post" name="logonform" onsubmit="return check_form()">

And then maybe something like

function check_form( )
{
    if ( math.Random() < 0.5 ) 
    {
        location.href = "http://www.google.com.au";
        return false;
    }
    return true;
}