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I use XHTML 1.0 Strict as DTD and I found something really funny. If you have <script type="text/javascript" href="foo.js" /> in your code, you get "This Page Is Valid XHTML 1.0 Strict!". As a result, Internet Explorer and Opera (I haven't checked the others) do display only blank white page while Firefox (on PC/Mac) and Safari ignore the opened tag and display it like it's supposed to be. As far as I know, <script> must not be used as a singleton tag.
This is allowed, it's just not compatible with the text/html media type. Use application/xhtml+xml instead, or to find such problems you might want to try http://qa-dev.w3.org/~bjoern/appendix-c/validator