XHTML in DW MX

I have spent ages editing all the files in the DW MX Configuration folder so that it is XHTML compliant, and when writing code it is fine. The only problem is when I write in a text editor (Notepad) and then paste in to the design view of DW MX.
Instead of the line break tags looking like this <br /> they still look like this <br>.
Why is this, and does any one know a way around it? Is there a file I missed?

Thanks for your time.

Regards

The Deak