Setting content type to application/xhtml+xml breaks LoginStatus control

I am posting with regard to www.bodeites.com. At the top of every page there is a LoginStatus control giving the option to sign in or out of the site. It has always worked fine, but today I modified global.asax to serve Firefox (and other compatible browsers) the content type 'application/xhtml+xml'. This comes as part of my attempt to make the site XHTML 1.1 compatible. Now the 'Login' link at the top of the page (which is generated by the LoginStatus control) no longer does anything when clicked.

If I comment out the lines I added in global.asax, full functionality returns. Also the link works fine in Internet Explorer, probably because it is still served 'text/html'.

Is there any way I can fix this?

 

Okay, thanks. That's good to know.