In the course of building test cases for xml-stylesheet processing in HTML5 (see https://www.w3.org/Bugs/Public/show_bug.cgi?id=14689 and https://www.w3.org/Bugs/Public/show_bug.cgi?id=15180), I discovered that none of the latest versions of Chrome, Firefox, IE or Opera implement clause 5 of "6.3.3 Requirements on User Agents Implementing the xml-stylesheet processing instruction" in CSSOM (http://dev.w3.org/csswg/cssom/#requirements-on-user-agents-implementing-the-xml-stylesheet-processing-instruction) or the parallel clause 4 in the next section.  I believe on the basis of a 3rd-party report that IE7 does do so.  Test cases can be found at
 http://www.w3.org/XML/2011/11/ssTests/test8.xml [vanilla case where SS is called
                                                  test8.css, served as text/css 
                                                  by Apache default,
                                                  works everywhere]
 http://www.w3.org/XML/2011/11/ssTests/test8a.xml [SS is called test8.foo, served
                                                   as text/css by explicit
                                                   setting in .htaccess,
                                                   works everywhere]
 http://www.w3.org/XML/2011/11/ssTests/test8b.xml [SS is called test8.baz, served
                                                   as text/plain
                                                   by Apache default,
                                                   works nowhere except IE7 AFAIK]

The xml-stylesheet PI spec itself says that the 'type' pseudo-attribute, whose value is 'text/css' in all the above examples, is only advisory, so this isn't exactly a bug _yet_, but I thought a heads-up was none-the-less appropriate. . .