I am using a clever piece of PHP code which asks your browser whether it prefers HTML or XHTML and then serves the correct MIME type. It also re-writes the XHTML to HTML if that's what your browser prefers.
The original code was not written by me - complete tutorial on how to do the same thing with your web page can be found on Neil Crosby's excellent website (called "Workingwithme.uk") here. WWM is an excellent resource for web developers and if you are interested you should certainly check it out!
First of all, are you using Microsoft Internet Explorer? If so, please see the information here and understand that I do not (because of cost) have any Windows machines of my own to test the page on, and having looked at it in the University public labs, I am aware that XHTML/CSS sometimes causes problems for Internet Explorer. I will still try to fix it, but it will take longer. Sorry :-(
I do do my best to test my site in as wide a variety of browsers as possible - I regularly test the site on Safari (both Jaguar and Panther versions), Mozilla (Camino, Firefox and the Mozilla suite itself), Opera and IE (Mac). I also test it in IE for Windows when I am able to borrow a machine for an hour or so.
If you wish to report a problem to me, could you please send an e-mail to owain@dsky.co.uk. If you are using an unusual browser (or Internet Explorer) a screenshot of the problem would be extremely helpful. On Windows you can take a screen-shot by pressing the "Print screen" key which copies the screen the to the clip-board and paste that into an image editor, and on Mac OS-X you press apple+shift+3 which will create a PDF file on your desktop.