[jQuery] bug in jquery ? css('border') returns undefined

[jQuery] bug in jquery ? css('border') returns undefined

I wonder why this doesn't work, either it's a but or a flaw in the documentation:
consider this code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "<a href="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd</a>">
<html>
    <head>
        <style type="text/css">
            h1 {
                background-color: #FF00FF;
                color: white;
                border: 1px solid red;
            }
        </style>
        <script type="text/javascript" src="../jquery.latest.js"></script>
        <script type="text/javascript">
            $(document).ready(function(){
                var mycss = $('h1').css('border');
                alert("hi : " + mycss );
            });
           
        </script>
    </head>
    <body>
            <h1>hello world</h1>
    </body>
</html>
...
mycss returns undefined instead of  '1px solid red'
Unless there is a good reason that i'm unaware of, this seems like a bug to me, no ?
Thanks
Alexandre