Install w3c-sgml-lib by entering the following commands in the terminal:
sudo apt update sudo apt install w3c-sgml-lib
Description:
w3.org DTD and catalog files
This package consists of all the definition files used by the W3C Markup Validator. It consists of DTDs and modules, entity files and XML and SGML catalogs. The catalogs are registered with the root catalogs.
Homepage: http://validator.w3.org/sgml-lib
Version: 1.3-2
Section: universe/text