While doing my recent rewrite of the coding in pointlessness, I found a part of this made my external CSS files (that contain all of my CSS) invisible to the W3C CSS Validator.
This part was me changing from using a "<link>" element to reference my CSS to using a "<?xml-stylesheet>" element (which is valid as per the XHTML 1.0 recommendation and remains valid in XHTML 1.1, which I use).
Because of this strange bug, after eliminating the "<link>" element from my XHTML 1.1 documents, the W3C CSS Validator no longer saw any CSS on my site. Reasoning that I might as well go all the way, I expanded the rewrite to trigger another bug in the CSS Validator.
If I don't specify language on my site, the CSS Validator works fine.
If I specify language only use an xml:lang attribute in my opening "<html>" element, it still works fine.
If I specify language using both the aforementioned attribute and a Content-language setting in my HTTP header, it continues to work fine.
If I specify language using only a Content-language setting in my HTTP header, the CSS Validator gives an error, saying "org.w3c.www.http.HttpInvalidValueException: Invalid content type." (despite this being a valid way of specifying language by itself as per the HTML 4.01 recommendation and not contradicted by either the XHTML 1.0 or 1.1 recommendations as far as I can see).
So, my XHTML coding is valid, my CSS coding is valid, but the combination of them fed into the W3C CSS Validator does not register as valid. Seeing as how I made sure my pages are within the relevant recommendations, does this make my CSS invalid? I say no, and so I will continue to say the CSS on pointlessness is valid.