As I blogged about some time back I was having some problems when running in XHTML Strict mode since Domino sent my stylesheet as text/html and not text/css. The solution was to rename the content field and the Content-Type for the document.
Another way to solve the same issue is to use a Web Site rule and replace the Content-Type header for specific URL patterns. You need to be running Internet Sites for this to work.
I now use this solution successfully for both JavaScript and CSS-files.