/etc/sgml/w3c-dtd-xhtml.cat is in w3c-dtd-xhtml 1.2-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | CATALOG /usr/share/xml/xhtml/schema/dtd/1.0/catalog
CATALOG /usr/share/xml/xhtml/schema/dtd/1.1/catalog
CATALOG /usr/share/xml/xhtml/schema/dtd/basic/catalog
CATALOG /usr/share/xml/xhtml/schema/dtd/catalog
CATALOG /usr/share/xml/entities/xhtml/catalog
|