This link works:

http://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fen.wikipedia.org%2Fw%2Findex.php%3Ftitle%3DMediaWiki%3ACommon.css%26action%3Draw%26ctype%3Dtext%2Fcss&usermedium=all

On that page, it says that you can also access it via this link:

"
If you would like to create a link to this page (i.e., this validation result) to make it easier to re-validate this page in the future or to allow others to validate your page, the URI is:
          http://jigsaw.w3.org/css-validator/validator?uri=http://en.wikipedia.org/w/index.php?title=MediaWiki:Common.css&action=raw&ctype=text/css
"

But that link doesn't work because the ampersand things are treated as part of the w3.org URL (and ignored) instead of the wikipedia URL.  The CSS validator ignores them and gives you the results for:

http://en.wikipedia.org/w/index.php?title=MediaWiki:Common.css

, which is not a CSS document.

It should not give you links that won't work.