i was writing XHTML 1.0 Strict document (served as application/xhtml+xml),
formatting them with CSS files. those CSS files were associated using the
xml-stylesheet instruction.
then i've found that i can use internal stylesheets in my XHTML documents [2]
i removed all the xml-stylesheet instructions, except this
 <?xml-stylesheet href="#stylesheet" type="text/css"?>
and inserted a style element with id="stylesheet" attribute.

the document was look like before, but the Validator can't validate it:
for it, the #stylesheet URI is the same document, not that style element (and
it's contents)

bbalint

[1] http://www.w3.org/TR/xml-stylesheet/
[2] http://www.w3.org/TR/xhtml1/guidelines.html#C_14
[3] http://free.srv.hu/s/i/singy/test.xhtml