Can a <hr /> element have a closing tag?

My colleague doesn't really know or understand html. Her job is to input information into the CMS and I've noticed she keeps closing her <hr /> tags like this <hr></hr>.

I've had a Google but I can't find anywhere that says this isn't allowed or could cause problems. I know it's supposed to be <hr /> but is it worth me telling her or is it unnecessary but valid markup?

NB The doctype for our website is XHTML 1.0 Transitional if that makes any difference.

EDIT

@Jeff had a good idea about validating. I used the following code and apparently this is valid XHTML 1.0 Transitional

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title></title>
    </head>
    <body>
        <hr></hr>
    </body>
</html>

Can a <hr /> element have a closing tag?

7 Answers