I get the following warning:

$ xmllint --noout --dtdvalid file:///usr/share/xml/fontconfig/fonts.dtd /etc/fonts/fonts.conf
/etc/fonts/fonts.conf:2: warning: failed to load external entity "/etc/fonts/fonts.dtd"
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
                                        ^

I don't see the point of this confusing warning since I just use the
--dtdvalid option, whose goal is to provide a DTD that is typically
different from the oone specified in the XML file.